There's a very informative new geeklist on BGG that lists the holy grails of game design.
Interestingly, the first item in the list is the desire for a playable civ game. Hmmm... Looks like Sands of Time has an audience ready and waiting for when it's finished!
There's a lot of good discussion of civ games in the geeklist linked at the top. Essentially it is a civilization building game where players start with a group of peopl eunder their control. Each player tries to develop their people technologically, intellectually, and socially while growing and developing the region in which the people live.
The starting and ending points are set by the specific game. It can be as sweeping as caveman to spaceman, or as focused as the late Rennaissance.
The main goal is the advancement of the people. I would contrast this with a game in which the main focus is the advancement of the land or regions in which the people live. Such a game would be more of an empire building game than a civ building game.
Hope this helps a little.