Linux is not a single operating system, but a family of operating systems, called "distros". What makes Linux unique is that most distros are open-source, meaning that the codes can be used by anyone to make their own operating systems. most are community maintained, meaning that what one person does to improve the operating system benefits everyone in the community.
Most Linux distros are free, as well as the mass majority of programs that are used on them. If you stick with the higher-end distros, such as Ubuntu, you'll find that most drivers are included with the operating system out of the box.
The file system on which Linux is written makes it very difficult for viruses and malware to propagate. As a result, there is generally not a need for anti-virus for a Linux distro that is alone on a network, however, any computer that is on a Windows network should have protection. Not for itself, but as an added layer of protection for the Windows computers on the network.
As stated before, Ubuntu is by far the most popular, but there are some other good ones as well:
Fedora-Core
Open SuSE
Sabayon
gOS
pcLinuxOS
Linspire
CentOS
Linux Mint
FreeBSD (not linux, exactly, but similar)
OpenBSD
Darwin
Just to name a VERY few. Most are available to be burned to a CD or DVD that can boot your computer and run as an operating system that were installed, without making any changes, or can be installed from the CD or DVD.
If you are even a bit curious, it is well worth looking into.