Here's a cool game I just played. There's an instructive moment at the beginning which demonstrates the principles of good and bad trades.
Here is the position: White's knight is under attack. How do you respond to this? Do you trade on d7? No, as that would allow black to recapture with a developing move, thus removing white's knight from the board while improving black's position.
Instead I played Nd3, which attacks black's knight and puts him under the same dilemna. If he trades on d3, I get to play Bd3, and his knight dissapears from the board. Note that by playing Nd3, (and by black taking on d3, not sure if there's other fancy stuff going on) black actually loses a tempo.
By the way, I played Na3 to protect against the threat of Qxd4, i.e. Qxd4 for white, then Nc2 + for black winning back the queen.
The rest of the game was total destruction as I had a huge development lead plus plenty of threats.
Here it is: