Capture-The-Flag, one of the most amazing ways to learn stuff while looking at computers all day.
In short: other people define challenges that can be solved by gainin access to some information called a "flag" (hence the name: "Capture the flag"). By providing the flag to the people organizing the challenges, one can probe that the challenge has been solved. How you gain access to the flag is up to you, (almost) everything is allowed.
The challenges themselves can fall into many categories (and some don't), but are mostly part of the reversing, web-exploitation, forensics, cryptography and binary-exploitation categories.