Technically, Linux is a kernel. a kernel is:
“The kernel is the master program (core) of the operating system. It manages devices, memory, swap, processes, and daemons. The kernel also controls the functions between the system programs and the system hardware.” Cisco’s UNIX curriculum v2.0

But in the the way I am referring to it as Linux is an operating system. Linux a free(normally) and open source copy of UNIX. Unlike Windows or Mac OS X you can freely modify the source code to Linux and do basically whatever you want to Linux. Linux is far more secure compared to Windows, it is not as easy for a user to break, the firewall, IP Tables is far superior, and there are essentially no viruses or spyware for Linux. There is also no Digital Rights Management in Linux, which is also very cool. I will add more to this later, but Linux is essentially an alternative to the Windows, Mac OS X, and O/S 2(not sure if anyone uses it anymore) operating systems. Also, Sun Solaris and BSD are fine operating systems, and I can give them to you also.
What is Ubuntu?

Ubuntu 7.10 and Xubuntu 7.10 are two of the many versions of Linux, and they are the versions that I recommend that you use. The desktop is Gnome and Xfce respectively, but that can easily be changed. Ubuntu Linux is one of the easier versions to figure out, as most of the hard work is already done for you. The other Ubuntu versions are Kubuntu, Xubuntu, and Edubuntu. The main difference is the desktop environment, it all depends on the one you like the most, or the way that it looks. A program made for any version of Ubuntu will work on all versions of Ubuntu. I will add more to this later, but this is a good summary for now. This website is currently inadequate, but it will be made better as time goes along. I am sure that many grammar mistakes have been made, and that I have probably misspelled many words.