Tennis: 250, 500, 1000, what does that mean?

I see these events tagged like that but what are they?

http://en.wikipedia.org/wiki/Association_of_Tennis_Professionals#ATP_World_Tour

Originally posted by Zamboner
http://en.wikipedia.org/wiki/Association_of_Tennis_Professionals#ATP_World_Tour

Ah, thanks. So I guess under that system, a player could be # 1 without winning a single grand slam. I think that just happened in women's tennis, Wozniacki, now #10 but was #1 with no grand slam wins under her belt.