"You can request a refund for nearly any purchase on Steam—for any reason. Maybe your PC doesn't meet the hardware requirements; maybe you bought a game by mistake; maybe you played the title for an hour and just didn't like it."
"It doesn't matter. Valve will, upon request via help.steampowered.com, issue a refund for any reason, if the request is made within fourteen days of purchase, and the title has been played for less than two hours. There are more details below, but even if you fall outside of the refund rules we’ve described, you can ask for a refund anyway and we’ll take a look."
The refund policy also applies to in-game purchases, but only for Valve games such as Team Fortress or CounterStrike. As long as it's two weeks after purchase and is not consumed in any way, you can get a full refund.
Pre-purchase is also included, the two weeks and less than 2 hours of game time will start after the games release date. Same applies for Downloadable Content.
Steam refunds is long overdue and will help build gamer's confidence on buying games online, especially with the popular Steam Early Access that's filled with games on sale but still in development. Steam has over 9 Million concurrent users as of March 2015 and is by far the most popular online service to buy and play games.
You can get all the refund details via Steam right here.