These are some sites that can explain better than I who he was:
Wikipedia Biography
The Heinlein Society
site:RAH - The Robert A. Heinlein Home Page
Quotable Heinlein (I don't believe this site has been updated but the search function is still there.)
Wikiquotes
My Heinlein-est View
In Honor of Robert A. Heinlein: Heinlein proposed some pretty radical ideas over the course of his writing career. I've taken many of them to heart and developed some of my own. My family finally got tired of listening to me and told to get my own soap box somewhere else. So now I get to throw them out to the rest of the world, thanks Blogger!
