Eagle is a program that you can use to make printed circuit boards.
It uses a visual format which is similar to, but not quite the same as, how you would normally draw circuit diagrams. It has a lot of snazzy features which make it very desirable for hacker/diy sorta people:
Eagle is not hard to learn, but it is kind of tedious at first. Trying to figure it out by yourself is really annoying, but with just a little help, everything goes a lot more smoothly.