On a VERY basic level, the main idea is to have a solid two-pronged pawn structure that converges onto the center, and also have an early activated light square bishop. If I were to guess when the Caro first came to be, it was probably some old GM who, unsatisfied by the French's bad bishop, devised the Caro-Kann Defense. At least that's how I see it. The Caro plays more like a system, and so it's probably not that great as a learning device for new players imho.
Caro-Kann ideas?

Most commonly I see the Advance variation. Anyway here is a game I ventured it it and won with:
Was move 5 bad or good then? I lost the Bishop but I weakened his Kingside and that ultimately lost him the game.

I'm not crazy about 2.e6 for Black. Black is voluntarily shutting in his Queen Bishop. One of the main "features" of the Caro-Kann is that in most lines, Black is able to develop this Bishop before shutting it in.
Also, no, I'm not crazy about 5...Bc5 for Black and exchanging this bishop. Black is already weak on the dark squares, with his three pawn moves. This dark-squared bishop is his "good" bishop. Any opening (for either color) that sees their good bishop exchanged on move 5, and locking in the queen bishop, must be suspect, in my opinion.
White should never have let both of his center pawns be captured like that.
Nice game. Thanks for posting.

The KIA with 2.d3 is not as good against the Caro-Kann as, say, the French, because Black is able to establish a classical pawn center quite early; his development is easy.
There is also 2.Nf3 d5 3.exd5 cxd5 4.Ne5 which you see some club players try to spring but it is gimmicky and not very good if Black knows what he is doing. What White is trying to acheive is a version of the Fischer style exchange variation where Nf3-e5 has been achieved for free and White does not have to preface it with moves like h3, c3, Bf4, etc. Here's a good response (after 8...Qc7! Black is equal because 9.f4? Nxd4 or 9.Bf4 Nd7!)
Possibly helpful: Starting Out: The Caro-Kann by Joe Gallagher (2002)
https://web.archive.org/web/20140627062050/http://www.chesscafe.com/text/hansen45.pdf

I play 3. f3 (Fantasy Variation), which often transposes to a Classical French (my old "double stem" line that I played for both sides) if Black sets up with ...e6.
3...Qb6 - your move
The KIA with 2.d3 is not as good against the Caro-Kann as, say, the French, because Black is able to establish a classical pawn center quite early; his development is easy.
There is also 2.Nf3 d5 3.exd5 cxd5 4.Ne5 which you see some club players try to spring but it is gimmicky and not very good if Black knows what he is doing. What White is trying to acheive is a version of the Fischer style exchange variation where Nf3-e5 has been achieved for free and White does not have to preface it with moves like h3, c3, Bf4, etc. Here's a good response (after 8...Qc7! Black is equal because 9.f4? Nxd4 or 9.Bf4 Nd7!)
What are the main ideas and themes in the Karo?