What is Wind?

Ok, i was once told that wind is created from the earth revolving! If so, how come there is not constant wind on ALL the surface of the earth? What makes some days more windy than others or why there's no wind at all on some days? I know it can depend on the terrain in which you live (on the beach, in hill country, on a prarie and ect.) but I'm just not completely understanding where it comes from I guess. I dont think I've heard the complete story, so if you know it....PLZ answer! Thanx Abunch