Maps are variable in detail and quality. Good topographic maps of your hiking area are a must. The scale of the map should be the largest possible, given the extent over which you want to roam. 1:100,000 is pretty coarse, but better than nothing; 1:50,000 (and its Imperial equivalent 1:63,360 ie. 1 inch to 1 mile) is a relatively coarse but eminently useable scale; 1:25,000 is typical and generally useful.
Maps are expensive - treat them well. A reasonable map cover, such as the one pictured complete with 'poor-mans protractor/romer' is a must. Consider laminating them, or even taking color photocopies in to field instead of the originals.
Many day-treks on nicely marked tracks have simple hand-drawn track maps. Better than nothing, but shouldn't be relied upon too much.
A map is a graphical representation of the ground. There are several elements that are important for a hiker. I'll try to restrict myself to the essentials.
I slipped in a term there - Scale. This is the relationship between the distance on the map and the distance on the ground. Scale is expressed as a ratio. For example 1cm on a 1:25000 scale map corresponds to 25000cm ie. 250m. A ruler is a useful tool to measure distance between points. If, say your start point and your intended target are 5.5cm apart, they will be 5.5*25000cm apart: 5.5*250m=1375m.
It follows that the larger the value on the right hand side of the ratio, the greater the distance a map unit represents. 1cm on a 1:100000 map is 1km.
Terminology is a bit confusing, but the 1:100000 map is at a smaller scale than the 1:25000 map. We have less space on the 1:100000 map to draw detailed objects, so features must be drawn smaller. I prefer to think of it in terms of fine-scale (1:25000) and coarse-scale (1:100000). We see more fine-scale detail on the 1:25000 map.
Grid lines and Grid References
Northings (left to right) and Eastings (up and down) are lines correspond to Grid North and Grid East (the term 'Grid' is important). Reading 'up' the map means you are looking at objects north of you, reading from left to right means you are looking at objects to the east. I'll explore what North and East actually mean below, but for the purposes of describing the position of a point on the map all we need is blind faith that we can do so using this Northing/Easting coordinate system. I talk about further details of coordinate systems below.
Now we have a coordinate system, we can tell other folks where things are by using Grid References. Let's use alt de Jucla as an example. The first rule is to read left-right along the map to the Easting (the up & down line) before the point. This is the 548000 Easting. Now for some mental arithmetic- we need to read these values in kilometer values. The coordinates are presented in meters, so we divide this by 1000 to get the value in kilometers - 548. Now our map is not covering 100km, so we can drop the '5'. The Easting is 48. Now read up the map to the Northing (the horizontal line) below. This the 34000 Northing. As above, we convert to km. The Northing is 34. There is no leading integer to drop. We now have a kilometer-scale description of where alt de Jucla is. It is in Grid Square 48 34. Somewhere. This is a four figure grid reference.
Of course we can do better than that. Our grid square can be further subdivided into 10ths ie. 100's of meters. alt de Jucla is 4mm to the right of the Easting - a bit more mental arithmetic shows this is 100m. Of course an easier way to calculate this is to read the value of the scale bar, or even easier still directly off the romer or a baseplate compass. So we can re-express the Easting in 100's of meters - 481. Now for the Northings... Alt de Jucla is 650m North of the 34 Northing. We can round to 700 (or 600, either works), so the Northing is 347. Where is alt de Jucla? Within 100m of 481 347. This is a six figure grid reference and is the staple of describing the position of a feature on a map.
A 6-figure grid reference tells us where the feature lies within a football field square. We can of course be smart-asses and further subdivide the grid into 10m blocks. In which case the position of alt de Jucla would be 4810 3465. This describes its position to 10m, and is a 8-figure grid reference. For those of you interested, 8-figure grid references are used by artillery, and for recording the position of buried soldiers on the battlefield. For the most part, 6-figure grid references are fine.
Map projection and coordinate systems
Let's backtrack a bit. With apologies to the Flat Earth Society, the world is not flat! Oh, and man did really land on the moon... With apologies to Plato, the Earth is not even spherical. It's shaped a bit more like a Tellytubby or, if you want to be boring, a bit of a dimply ellipsoid. The kind-of-round earth has been known since antiquity - Columbus didn't invent the idea. Pythagoras deserves the credit. It is pretty hard to draw a map of something resembling an orange. It is doable, but has the problem that distances at right angles to the axis of the sphere change with how far you are away from the Equator. You need to walk much further to move 1 degree East at the Equator than you will at 45 degrees North. So Euclidean geometry and Pythagoras' formula won't work. However a spherical object can be projected into 2-dimensions, albeit with some distortion. The best known projection to the layman is the Universal Transverse Mercator or UTM projection, which kind of turns the Earth into an unrolled toilet paper tube with a slightly scrunched top and bottom.
Many, but not all, maps a hiker will use are based on the UTM projection. This distorts the Earth's surface into something more closely representing a Euclidean surface ie. 1 unit Longitude is the same length (or close enough) as 1 unit Latitude. The example Andorra map uses a conical Lambert projection. For the most part, the projection of hiking-scale maps is not too important - it only becomes important if navigating a ship or passenger airplane or ballistic missile. So now you can use your trusty ruler (and Pythagoras's theorem) to directly measure the distance between points. Protractors with Romers (marked scale) and most baseplate orienteering compasses already have useful graded scales for measuring distances from grid intersections. Here's where things can get a bit tricky.
For convenience, the world is divided into a somewhat arbitrary grid division. Now every grid has a start point - a 0,0 point. These differ between maps, and North vs South Hemispheres, and Polar vs low latitude regions. For example, the UTM series starts from a hypothetical point - a False Easting and False Northing. So although the units of the grid are meters, they aren't necessarily meters from a meridian such as Greenwich, or the Equator - the obvious 0,0 point. The main reason for this jiggery-pokery is to avoid problems with accounting for negative measurements when crossing map boundaries. At a hikers level, well a grid system is a grid system. Who cares? Geometry doesn't depend on different 0,0 start points. This does become important when trying to compare positions on different coordinate systems and, of particular importance to the hiker, when setting your sat nav so it aligns with the map. The example maps I use of Andorra are NOT in UTM - they are European Grid, with a different 0,0 point to UTM. This is important if using satnav as a navigation check. With a bit of arithmetic, you can work out the difference between the two from a known point - they only differ by a constant in East and North - but it's best to work out this fudge-factor before you set out.
Compass rose, declination and map information
The map information and legend combined contain all the useful information particular to the map. In the map information you will find the coordinate system, the date the map was made, the datum the map is set to (see Satellite navigation), and the scale.
The compass rose shows "North". However, there is more than one "North". No wonder Santa always appears confused. We have already met Grid North, which is the North the map is aligned with. This is occasionally, but by no means always, also True North – the North you would expect if the lines ended at the North and South poles respectively. This is a wee bit of a simplification, as the map projection enters the equation when the map covers a larger area. For a hikers purposes, we can ignore which north the grid refers to. There is also Magnetic North. This is very important to the hiker because Magnetic North is what the compass will point to, and is invariably different to Grid North. The magnetic north pole is generated by metallic elements in the (moving) earth's core, not the axis of rotation.
Why, you may ask, aren't map grids just aligned to Magnetic North? Because, Grasshopper, Magnetic deviation changes from place to place, and maps of adjacent areas would no longer line up. Magnetic deviation also changes annually. The Magnetic deviation is measured at the time of the map survey, and the navigator has to add each annual shift to get the correct deviation for a given year. The deviation of magnetic to grid north is provided on the compass rose, together with the deviation and annual declination. Here is where that damnable earth's magnetic core becomes a pain in the backside. From just east of Western Australia to the west coast of the US the deviation is to the East (right) of grid North. This is particularly important for those who learn to navigate on one side of the world and the appropriate mnemonics, then blindly follow the same convention on the opposite side of the world.
Now we can orient the map. We are going to use a compass, but first we need a bit of arithmetic. Magnetic north deviates from grid north, so we need to account for this. From the compass rose, we see that the difference between Grid North (in Catalan abbreviated to NP) and True (Geographic) North (in Catalan abbreviated to NG) is 0° 31' 12" W. The difference between True North and Magnetic North (in Catalan abbreviated to NM) is 0° 03' 14" E. This is a really annoying feature of this map - we generally don't care where True North is, but the cartographers make us do an extra calculation just to show how clever they are. So, we have to add 0° 31' 12" W and 0° 03' 14" E together to get the Magnetic to Grid deviation. In this case, 0° 31' 12" MINUS 0° 03' 14" gives us 0° 27' 58" W as our deviation. However, this deviation was correct in 2013. The annual declination is 0° 07' 25" E per year. So in 2016, the extra declination is 3*(0° 07' 25" E) = 0° 22' 15" E. 0° 27' 58" W MINUS 0° 07' 25" leaves us with the magnetic variation in 2016: 0° 07' 33" W.
Hang on Mr Fussy-pants! I hear you say. We are lucky to measure direction to within a degree!. Yes indeedy... On this map the difference is little more than 1 tenth of a degree, so you may as well ignore it. However, this is not true for all maps. If you really just want to do the calculation in your head, you can round to the nearest degree.
Now the orientation side of things. Ensure your compass is level, the needle isn't still swinging wildly, or pointing toward your to-close rifle barrel, and align the Eastings (the up-and-downs) with the compass needle, after correcting for magnetic deviation (in this case zero).
Key and marginal information
In the key, you will find colour codes and symbols for vegetation and other features such as roads, escarpments, lakes and so on. Symbols often differ between maps, and different cartographers or institutions place different levels of detail about things such as vegetation types. You need to become adept at sorting the wheat from the chaff so to speak. I, for example, find the British Ordnance and Survey maps too cluttered - but if you grow up with the things you won't notice it. I just mentally filter out the clutter.
A contour line is a line that joins adjacent points of equal elevation. Put simply, if you are walking along a contour line you are walking on the flat. If you are moving across contour lines from low values to high values, you are walking uphill. Conversely if you are walking across contours from high values to low values, you are walking downhill. If the high to low values are changing very rapidly, you've walked off an escarpment. There is (as always) an exception to the high-low value convention. If you are below sea level, for example Death Valley, the reverse will apply. The contours represent height above sea level, based on the map datum. Contour lines are also the singularly most important thing to have on a map - they describe the shape of the land.
The marginal information on the map provides the contour interval. On my Andorra map, this is on the scale bar, in the OS series in the 'Heights and natural features' legend. This may vary between maps. The OS Hastings and Bexhill map (pretty flat) shows contours at 5m intervals. The Andorra map (mountains!) in 10m intervals. This will also change with map scale.