Linux is like the core set of instructions that make up the operating system. Companies and people take the linux core (called the kernel) and create their operating system around it. There are many "flavors" of linux. The different linux'es are called distributions. The linux world is generally what is called "open source" where anyone can use anyone else's work as long as they give them credit. The kernel is only 1 part of the entire operating system. When you get a linux distribution it is made up of many parts. such as the kernel, a GUI (graphical user interface), device drivers and managers, software installers, office applications.... and so on....
there is a ton more to this...... a favorite website of mine is www.distrowatch.com they keep track of all the linux distributions and what is going on with them
for a home user looking to get into linux on an old pc I recommend one of these:
SUSE
Fedora (used to be redhat)
Mandrake
Linspire
Last night I installed the newest version of SUSE 9.1 You can download a disc image for free! all you have to do is burn it to a cd and install it. also ebay is a great way to get it cheap.
so far SUSE 9.1 is awesome. the thing about these distributions is that they are loaded! they come pre installed with everything you will most likely ever need. office programs, drawing programs, cd burning, music....... on and on. All FREE!
obviously i could go on and on about this.... i really like the linux stuff...... but i have not 100% left microsoft..... but that is just a matter of time.