What's the dividing line for a cult versus a religion?

I was listening to NPR and the story of the LDS publication acknowledging that Joseph Smith had at least 33 wives, at least one of them as young as 14 got me wondering. Was it a cult at one point that later began to be considered as a religion? (Come on, there are so many aspects to the story of Joseph Smith that make it sound like someone just waiting to pass out the cool-aid.) Is it numbers of followers? When do cults become legit? Did every religion start out as a cult?