What is an unlimited internet plan on a smartphone?

I was on the internet on my Samsung Galaxy S, and this dude said, If you go over your limits on internet you will have a big bill (my parents ) lol, anyways IS that true? Is there really a limit on the internet you can use? And he said you can get the unlimited for like $100 a month. (that's alot! geez!) I have t-mobile by the way. I think the dude has verizon, but he used to have t-mobile so he said he knows what he's talking about. Thanks for answering :)