What's the difference between a cult and a religion? (serious question, don't flame)

So today I was taking a bus to the mall and on the bus there was some fundamental trying to get signatures to close down some family planning center in town. He came to me and I turned him away, he started to preach to me in an attempt to guilt me into signing and i just said "I'm an atheist so preaching doesn't work on me and I already said I'm not interested". I'm sure can imagine what happened and eventually he said something like "You people think we're all the same, I bet you think Christianity is just a cult like those other so called religions (in reference to Muslims)" and I asked "Well what's the difference?", he stared at me and decided to walk away after saying "I don't have time for this."

So it got me thinking: "What is the difference." I mean a few years ago people would have easily labeled something like Scientology under "cult" but now it considered a full blown religion. And of course by cult I don't mean your "drink the punch" cults. I mean what is the real difference between them, if any?