...I can't tell if you're joking or not (tons of hype for like the last two years) so I'll try to give the basic version of it.
You start as a multicellular organism, growign larger and larger and evolving new parts until you evolve enough to grow legs.
Now you're a creature (either herbivore or carnivore depending on your decisions during the cell stage), and must eat and kill/make friends to survive. Once you've evolved enough, your creatures become intelligent and form a tribe.
At this point it feels kinda like The Sims somehow, cept The Sims didn't tend to let you go to war... But anyways, you must now either befriend or kill (noticing a trend?) the other tribes to evolve into a City.
City = Civilization it seems, cause it boils down to "make more $, buy out other cities, or attack them to convert them."
Then enters the space stage, which is so hard to describe I'm going to point at Wikipedia and have you look there
Or look for the 30+ minute videos on youtube, theres some of the Game Dev converences on it; theres two areas that were in videos that I don't believe are in the actual game - in the videos theres a stage between cell and creature which is missing (think Otters and Dolphins), and theres a plant developer that isn't in the game yet either. As a note though that plant development thing is supposed to be a separate release sometime later.
But yeah. I've clocked 22 hours since Friday not including today. We've got an addict right here *points to self*
Edit: Doh! forgot the link to wiki.
http://en.wikipedia.org/wiki/Spore_(2008_video_game)