Explain the data package please!?

I currently have the Env3 without the data package and I want to upgrade to a smart phone. I was looking at the Droid Incredible 2 and I think that's what I'm sticking with but I'm confused about the data package. If I want to get the data package, does it it come with all the web surfing, texting and calling all for $30? Or is the $30 added onto my original monthly payment?