Yeah cube is a FPS. Its not build to be very nice but to show off a new render concept. Involves building a map consiting of cubes

. Sauerbraten is something like cube2. Its much improved and much nicer.
Cube featured a simple singleplayer mode and the 'normal' modes from online play. But as far as i know only instagib was played online.
Both games feature a ingame edit mode for the maps and (very cool) you can even edit a map in teams. Just start your server in edit mode and let others join.
Sauerbraten got much advanced features as bumpmapping, normal and gloss maps so it can look very very nice as you can see on the screenshots.
But as i said i never player cube for real and up to the last release i thought sauerbraten was still beta.. Will be interesting to watch this game mature.