Valve has unveiled its new Beta Steam Trading And Exchange Service, which once enabled allows you to trade with other gamers through Group Chat, and trade with anybody in your Friends List who’s also opted in to the new Beta service.
Currently in its Beta version only Team Fortress 2 items and Steam Gifts can currently be traded , but Valve are currently looking at adding more games in the near future. The new service allows you to trade games you’ve purchased from the store as a gift, or received as an Extra Copy and can be used to trade for other Gifts, or for items within the Team Fortress 2 game.
To start trading sinply right-click on their name and select “Invite to Trade” from the dropdown menu. Once your friend has accepted your offer to trade, a trade window will then open up. Don’r worry the first time to start a trade with someone a tutorial will guide you through the whole process from start to finish, step-by-step.
Source: Valve :