A few select commands are considered "cheats", and can only be activated under certain conditions. To activate cheats, type:

/sv_cheats 1
from the console. From there, load any map using the /devmap command. For instance:

/devmap q3dm15
will load "Demon Keep" with cheats enabled. You can only use cheats when using the "/devmap" command -- using the standard "/map" command will not allow cheats.
Once you've got cheats enabled, some other useful commands:

/god - "God" mode. You are invincible and cannot be hurt by weapons.

/noclip - "no clip" mode. You can fly through walls and floors, and basically go anywhere you want in the level. (Great for taking screenshots.)