Once a game is in your steam library, you're stuck with it. Games that you buy as a gift or receive in a trade are placed in your 'inventory' which is different from your library. Your inventory can be traded on steam in an official trade window by inviting your friends to a trade.
Some people are trading keys. Steam keys are codes that are redeemed for a game on steam bought elsewhere (like amazon). People often get keys they don't want in bundle offers - like I got Civilization V and XCOM keys (my pc has got no chance of running them) from Amazon when I bought Civilization IV. The downside to keys is they go straight to your library - you can't trade them in the official trade system, so you're stuck trading codes.
...I don't suppose you want to trade for an XCOM or Civilization V code?
Can someone tell me how this all works. I have a game dungeon siege that I don't want. Can I trade that? Black Ops 2?