Current versions (i.e. v7) has 2 modes of running.
If it is run as a non-admin, it’ll install a service called UAService7.exe, which works in ring 3 mode (not so bad, and works around one of my pet peeves, i.e. requiring administrator mode to install, or worse, run the game.)
If, however, you haven’t wizened up to the benefits of not running as an admin, it’ll install a driver in ring 0, which basically gives it full access to wherever it might want.
Can require online access to authenticate the game during installation and/or when you start the game.
Can limit the amount of times you install the game.
Will probably only work as long as the games publisher keeps the authentication servers online.
For more information: http://en.wikipedia.org/wiki/Securom
My personal theory on this whole install limit thing is to cut down on second-hand sales.
Games using SecuROM v7: