How do you count cards?

I have 21 (the movie), and i was wondering... how do they do that?? Look at this pic: http://howto.wired.com/mediawiki/images/Cardcount_corrected.png what is all that +1, and -1? and what does that mean? i thought blackjack was just about you know, addition... lol.... what i really want to know is how they count the cards, and how they find the count, and and all the +1, and -1 like the picture Lol.... i really appreciate the answers x]]