Steam is developed by US based developer and digital publisher Valve (Half Life 1+2, Counterstrike, Left4Dead) and was one of the first download and publishing platforms.
After a somewhat shaky start - we all remember the big discussions around the forced distribution of the local Steam client with Half Life 2 - it developed into the market leading provider of digital games. As of some unconfirmed statements it is said to hold up to 70% of the games download market!
In addition to the download and publishing aspects the Steam platform pioneered a number of features like automatic patch handling, a seamless installation, achievements and community features. However, to use all this exiting stuff you need to install and run a local client that comes with its own benefits and drawbacks. Still, Steam offers one of the most comprehensive and streamlined online shopping experiences for download games on the web.