Stack Exchange is a network of question and answer Web sites on topics in varied fields, each site covering a specific topic, where questions, answers, and users are subject to a reputation award process. The sites are modeled after Stack Overflow, a Q&A site for computer programming questions that was the original site in this network. The reputation system allows the sites to be self-moderating. User contributions are licensed under Creative Commons Attribution-ShareAlike 3.0 Unported.