Codeforces is a Russian website dedicated to competitive programming. It was created and is maintained by group of sportsmen from Saratov State University led by Mikhail Mirzayanov. Codeforces provides to all users following main services:
participation in the short (2-hours) contests, so-called "Codeforces Rounds", held about once a week;
ability to solve problems from previous contests for training purposes;
"polygon" for creating and testing problems;
kind of social-networking by using of internal public blogs.