We are of course all familiar with basic maps, and I am sure we can all agree that the top of the page a map is printed on represents North. Likewise, South is down, East is right, and West is left. A compass has these directions too - of course - but the difference is how a compass actually works. The red end of the needle on the compass points to magnetic north, where as north on a paper map is true north. True North is the geographic top of the earth. However magnetic north - which right now is located in North Western Canada - This difference is called declination.
We need to take declination into account when ever a map and the real world interact. If we are using just a map, we don't have to worry about declination. Likewise, if we are using just a compass, we don't have to worry about declination. It's when we are using both together that we need to be careful. While it isn't difficult, it is where most navigational errors occur.
Declination is recorded as a number of degrees, and either east or west. For instance, where I live the declination is 8º west. What this is saying is that the compass is pointing 8 degrees west of true north. So I have to account for it. If I am going to follow a compass bearing of 210º I want to add 8º to account for declination so I really need to follow a bearing of 218º. Some compasses - like the suunto m3d that I use - can set the declination so you don't have to do math. I know I need to add the declination because it is a WEST declination. If it was an East declination I would subtract it from my original bearing of 210º. This is where the rhyme 'East is least, West is best' comes from.
We can find declination in the margin - or key - of our map. It will generally look like this: