Boot Camp allows Mac users to install Windows on a hard drive partition. On Apple's latest 13-inch MacBook Pro with Retina display, as well as the most recent models of the MacBook Air, only the 64-bit release of Windows 8 is supported in Boot Camp. All three of the new laptops were announced earlier this month at Apple's "Spring Forward" event.
If you own one of these machines, and need to run Windows 7 on them, you'll need to turn to another solution, such as VMWare Fusion or Parallels. You can find the full list of Boot Camp-capable computers and which versions of Windows they support on Apple's Support site.