Well, you can learn to rely on a magnetic compass if you are on a steel ship, although this is no longer often done. When you are working with a tiny little compass that's held by hand, the differences are probably not going to matter. But as much as you can, learn about checking your compaass and do not move the metal you carry around too much. And learning the variation for your area should be relatively simple.
Deviation would be the error that's due to what you have on or around you. When steel ships were first constructed, there were issues, because the compass readings changed, because of the steel in the ship. Sailors learned how to correct for this error, and you will see near compasses on ships a deviation card. the card basically states that the last time it was tested, the compass had so much error when on various headings due to the magnetic influences of the ship. The ship is a big hunk of steel and that steel does not for the most part move that much (although, cargo moves). So, if you have a certain deviation today on a certain heading, you are likely to have the same deviation on the same or a similar heading in the future. However, if is important to regularly check your deviation and variation.
Variation is due to changes in where all magnetic compass point to from true north. It changes across the surface of the Earth. If you looked at a navigational chart, it would have a circle indicating true and magnetic directions. Also, it would state the variation and how much that variation changes by each year.
Edit: For simple ways to check your compass for errors.
1. Sunrise. The sun comes up in the East. The further it is from the Equinox, the less true this is, but if you hold you compass at sunrise and turn slowly around checking to see how the headings change. The heading of north, east, etc. should not change. It should always point to what it seems is north, any changes shows a change in the error, so that's deviation.
2. Same, but at sunset, and using west not east, obviously.
3. Noon, or Local Apparent Noon ("LAN")as you would hear in a celestial nav class. At the time of day when the sun reachs its zenith, the highest point it is in the sky all day, the sun must be either directly north or south of you. Now, if you do not know the time of LAN, you could waste time sitting to figure it out, but if you do know it roughly, it will tell you due north or south.
4. Polaris. It's pretty much due north. So, if you can see it, you can tell how much a compass is off.
If you want to read about compass correction:
http://www.irbs.com/bowditch/pdf/chapt17.pdf