Internet Chess Servers
An Internet Chess Server is a server out on the Internet where you can log in and play chess with people from all over the world.
You can also play against computers, watch other users play, or just hang out and chat.
Many of the chess servers on the Internet today are distant descendants of the original ICS written by Michael Moore and Richard Nash in 1992.
Today's ICSes run completely rewritten code, but they have kept enough compatibility that many chess clients still work with all of them.
In particular, you can use XBoard and WinBoard as graphical user interfaces to all ICS-compatible servers, and you can use Zippy to hook up certain chess engines to them as automatic players.