Simply called Steam Trading, the newly launched beta program allows users to exchange in-game items and gifted game titles between themselves. At launch time the program only supports Team Fortress 2 items and games purchased as gifts in the Steam store, but Valve says they are working with developers to add more games and the company plans to add Portal 2 in-game items to the beta shortly.
Once customers purchase a game as a gift (or receive it as an extra copy), they may trade it for the games or virtual items of other users. Any "unopened" gift is fair game for trading, with no restrictions on territory. Trading a Steam Gift is very similar to sending a gift to someone; just that once it is traded that user can then either trade it again, or open it for themselves. There are no restrictions on territory.
To participate in the beta, users must open the Steam client and select the "Steam Trading Beta" option from the settings menu.
For more details checkout the Steam Trading FAQ's.