For a while now you were able to open up Team Fortress 2 and trade weapons and items between two players, however the trading ended there. You had to open up the game to trade, and you could only trade in-game items for other in-game items. However the new Trading System beta, which originally debuted a few weeks ago, integrated the trading system directly into Steam. Now you can launch a trade right from a users profile or a chat window and trade in-game items from Team Fortress 2, Portal 2, or Spiral Knights for other in-game items, as well as extra/gift-copies of games. The Trading System is out of beta and fully released, all you have to do is restart Steam and grab the new update to enable the feature.
The Trading System has been integrated into the Steamworks SDK, meaning that other titles can leverage the system. For now there have not been any announcements about other games that plan to use the feature, but its only a matter of time until another game utilizes it. All users also have a inventory which you can view to see what items they have available.
This new system not only expands the in-game economy of Team Fortress 2 and other titles that support the feature, but also helps avoid some of the less user-friendly policies of Steam. Previous if you gifted a game to a user who was in another country where that game was not available, Steam would either block the trade or possibly suspend your account for violating their policies. And if you accepted a gifted game from a user who made the purchase with a stolen account or credit card you also risked suspension. With this new Trading System Steam can simply revert trades made with stolen accounts or payment methods, and avoid any sort of territory restrictions, which makes trades safer for users.