The Steam client is similar to a browser, with it's own look and feel.
Steam is also owed by game producer Valve, so of course, their games are available here. They boast such titles as Half-Life, Half-Life 2, Left4Dead, Left4Dead2, Portal, Team Fortress 2 and Killing Floor. All of these titles with the exception of the L4D franchise are not only available for the Mac but they now support SteamCloud, which saves games online so you can access your saved games from any Steam-enabled computer.
In addition, they also support SteamPlay. That means you can play your game from any PC or Mac. That's right. Buy it on the PC, play it on the Mac, at no extra charge. I know I am using a lot of italics here, but I am really excited. I had never played these games before, and Steam gave away free copies of Portal for Mac and PC users during the release of the Steam client for Mac. So of course I downloaded it.
*insert sound of revelation here*
Then I was persuaded to pick up the Orange-Box, which in addition to Portal, comes with Half-Life 2, Half-Life 2: Episode 1, Half-Life 2: Episode 2, Half-Life 2: Lost Coast and Team Fortress 2. I had heard about Half-Life but never played it. For the most recent past I have been playing adversarial games online and had not played single player for a while.
Then I tried Half-Life 2. 25 hours later, I was hooked.
All in all, if you like games, Steam is a must-have.