A bit of a better explanation then everyone else..
RedHat linux is a commercial brand of linux, aimed at enterprises and developed by Red Hat inc, and is generally aimed at the buisness market. It is an Operating system based on the linux kernel. the current release of Redhat Enterprise linux (RHEL) is at version 5.
RedHat sells the product and support for it.
While RHEL isn't available to the public for free, RedHat offers a free version known as "Fedora." Currently at version 6, Fedora is much like a continuous beta of RHEL.
Other Linux OS's base their product on Redhat, such as the CentOS project, which aims to be like a free version of RHEL.
If you intend to switch to linux, Redhat might not be the best place to start. Try something like Open SuSE, PClinuxOS(I hear) or Ubuntu