The 'real' version of Portal is included in Valve Software's Orange Box, the all-in-one edition of Half-Life 2. The idea is similar to that of a standard first-person shooter, with one big difference: in your inventory is a gun that can shoot 'portals' into walls. Shoot one portal somewhere and another somewhere else, and you can step through the first and out of the second.

The Flash edition of the game is much simpler (as are the graphics, for obvious reasons), but the principle is the same. It's reminiscent of old-school 1990s adventure games in many ways - especially Flashback, which were disappointed no-one has yet converted to Flash.