Just out of plain curiosity, which came first, Stack Overflow or Stack Exchange? I'm led to believe it was Stack Overflow, simply due to the similarities to Stack Exchange, but I may be wrong. So, which one did come first?
Stack Overflow was the beginning. It was the original, first site.
It being popular, it spawned a few other sites - Server Fault and Super user. Which then started spawning other Q&A sites.
This evolved into the network of sites you see today - the Stack Exchange network of sites.
You can see the evolution if you start reading the Stack Exchange blog from the start (and listen to the original set of podcasts).