Linux Comes Packaged In "Distributions"

The Linux kernel, by itself, is not useful. It provides services for applications to take advantage of, but without applications there are only a few things it can do. Fortunately, a wide variety of groups, organizations, and companies have combined Linux with other applications in the form of "distributions".

A "distribution" is a combination of the Linux kernel, applications to run with it, utilities to install it on a computer, and utilities to manage the system once it's installed. Distributions vary in the options they offer, the utilities they provide, what commercial software comes bundled, price, and so forth.

A few of the more popular distributions are:

Red Hat - popular for its ease-of-use, graphical utilities, software management system, and bundled commercial software.
Debian - a non-commercial distribution, Debian concentrates on using open-source software whenever possible. Popular also for its advanced software management system.
S.U.S.E. - produced in Germany, this distribution is very well liked by some for its administration tools, comprehensive application set and thorough documentation.
Slackware - one of the oldest distributions, it is popular for its easy maintenance.

It's worth noting that, for all their differences, the Linux distributions are all pretty much interoperable. There is a standard for how directories are set up, and standards for things like shared libraries (DLLs) and so forth. This means that almost all Linux programs can run on any distribution.

Individual applications usually come in "packages", much like ZIP files. The most popular package format is "RPM", developed by Red Hat. Almost all distributions either use this format natively, or offer tools to automatically translate and install RPM files. Choosing a distribution is really a matter of taste.