Incase you were not entirely clear on what waffles said, here is the breakdown:
StackOverflow, Serverfault, Superuser, and Meta StackOverflow were created by Jeff Atwood and Joel Spolsky. They all run on the same software.
After these sites became popular, and after many people asked how they could run their own Q&A site with a similar format, the owners developed the existing software into a product called StackExchange. We'll talk about that later.
The software behind StackOverflow is not open source. Many people don't like that. This is because everyone on the internet expects things to be free. Unfortunately, Jeff and Joel buy into the idea that when you put effort into making something that gets really popular, you deserve to eat, sleep in a house, and generally be able to make a living.
Of course, many people do not subscribe to this theory, and they have attempted to clone StackOverflow. Some people claim it can be done overnight, and some people have actually done things that are similar to StackOverflow. However, they do not have the polish or the community that StackOverflow has, so they are not really that great. You can read Jeff's post on this subject, and you will quickly realize why these don't work.
Now, the good stuff:
StackExchange is a site that allows you to pay money for your own Q&A site running on very similar software to StackOverflow and the like.
You get reputations, up votes, and bounties. You do not get the ponies. The ponies and bacon are reserved for Meta, My Personal Series of Rants, and OMG Ponies.
Currently StackExchange is building up its features and polishing the rough edges to make it ideal for starting up communities.
So what have we learned?
- Ponies and Bacon are Reserved for Meta StackOverflow Rants, as well as April Fools Day Pranks.
- StackOverflow, Serverfault, Superuser, and Meta all run the same software.
- This software may attempt to be duplicated, but ultimately fails
- StackExchange.com is the best way to get a Q&A site similar to StackOverflow.