January 1, 2008 25th anniversary of the day when ARPAnet, an early computer network, switched to communicating via the TCP/IP set of networking protocols. TCP/IP is the set of protocols that has been used by the Internet ever since, making it possible for computer users from all over the world to communicate, no matter what brand or type of computer they're using.

This article from History of the Internet explains it all in more detail (and with as much technical explanation as you might want). History of the Internet is a fun and free way to learn about how the Internet came about and the technologies that make it work.