Linux is an open source operating system. It is composed of a Kernel (the central controller for the Operating System developed by Linus Torvalds), the GNU utilities, and a variety of applications. There are several linux distributions (aka Distros) available that have different features. If you're new to Linux (both the operating system and applications), check out a live disto like Knoppix or Ubuntu that can be run directly from a CD-ROM/DVD-ROM (or RW) drive. Links to most distros can be found on Distrowatch (http://www.distrowatch.com).

Documentation is available with the distribution. I also recommend looking at some of the documentation available from TLDP (The Linux Documentation Project - http://www.tldp.org). Particularly NAG (Network Administration Guide), SAG (System Administration Guide) and LAME (Linux Administration Made Easy).

Other documentation exists on TLDP as well. And there are a variety of Linux / Unix resources out there on the web...usually the home of your distro of choice.

Tim