The idea behind 1. e4 c6 is to play d5 on the next move and so try to remove the white e4-pawn, thus undermining white's influence in the center.
This should be compared to the scandinavian (1. e4 d5) which has the drawback that when white takes on d5, black has to recapture with a piece, and with the french defense (1. e4 e6 2. d4 d5) when black often has trouble developing his c8-bishop because it is blocked in by the e6-pawn.
So the main principles behind 1. e4 c6 2. d4 d5 are:
1) to try to undermine the e4-pawn while being able to recapture on d5 with a pawn if white plays exd5;
2) to be able to develop the c8-bishop to f5 or g4 before playing ..e6.
The theme that recurs the most often in the caro-kann is that black will reach a very solid position with almost no weaknesses, while white has an initiative but cannot find a way into black's position and will lose trying to break iron with his hands.
Caro-Kann
If you want to learn to play the Caro-Kann and memorize the moves by heart join Caro-Kann lovers. There we discuss the Caro-Kann through the forums and play team matches and vote chess. During team matches we play the Caro-Kann. If you have any questions contact me!
The main idea behind the Caro-Kann is to bore the white player to death, so that he falls asleep and loses on time. 
The Caro-Kann is regarded as stodgy and drawish by many players - and not without good cause! It, like the French, is a defense that requires Black defend before bringing his own counterplay to bear. Not my words, but the words of Nick de Firmian, author of MCO. Still, it's a weapon that I have a great fondness for.
The Main Line (Capablanca Variation) is what you should probably play. Don't delve too deep into the theory though: this is just an example of how many games can go.
More often than not you'll face things like:
1.e4 c6
2.Nf3 d5
3.exd5 cxd5
3.d4 Nf6
And Black has a good game ahead of him or her.
1.e4 c6
2.Bc4 d5
3.exd5 cxd5
4.Bb5+ Bd7
5.Bxd7+ Qxd7
Black has a pretty nice game here as well. Again, don't get too devoted to the theory - people often will diverge from the book, more often unintentionally than not. Good luck! If you have any questions, and it might be a little arrogant of me to say this, but I'm sure I can answer them. Drop me a message or add me to your friends!
I am interested in playing the Caro-kann. Can someone tell me the main principles in the opening? Most important thingsto keep in mind?