Cool question....
I've been back and forth on this topic my whole life. I was raised in a church that eventually betrayed me in the worst way. I was taught the beliefs of this particular religion and had a bit of a hard time letting go of some of those beliefs when I started thinking for myself. As I got older and had more experience in life - experience not influenced by this loony organization - I came to understand a lot more. Mainly that organized religion is a total fallacy.
I do believe in God. He has made His presence known to me in a few ways. I believe there is much more to our existence once we die.
I don't think we float up into the sky and grow wings and play harps, like Lauren joked about. :-D I think that all the things we wonder about in this life; why are we here, what is our purpose, what is the meaning of life - I think these questions are answered. I think our soul, or our minds (I'm not sure what to call it) become aware of everything we didn't know in our mortal lives. The brains that we use only a small percentage of are freed from our mortal bodies and can realize everything. We attain a new state of being. Maybe not a physical being, but an aware being, nonetheless. Everything God has meant for us becomes our reality. We are reunited with our loved ones who have passed on, we understand why God made us, what we've messed up in our mortal lives, we become better than what we were as people on earth.
I've never tried to put into written words what I think heaven or the afterlife is like and I'm not sure I've done a very good job describing what I believe here. I hope it makes some kind of sense to somebody other than me!