Operating systems are the software that makes your computer work (or crash). Typical, well known examples are Unix, Linux, Windows, Free/Net/OpenBSD and so on.
Operating systems are one of the most amazing issues with computers and computer technology. I will strongly recommend you to read Tanenbaums excellent book "Oerating Systems, Design and Implementation" (the book that inspiered Thorvald Linus to start developing Linux).
Below are some links to less known operating systems, and operating systems resources.