A distro is a collection of software grouped with an installer, and usually has a maintainer that produces updates. Some are based on a package-system (RedHat's RPMs, Debians DEBs, etc), and others are based on source files.
The best way to try Linux is to get a copy of Knoppix... << www.knoppix.net
>> It runs straight off a bootable CD, and doesn't install anything to your hard-drive. Since it runs off the CD, it's a bit slower than a normal installation. You might be able to save configurations, but if you don't, they are lost when you shutdown. But, it's still a great trial version.
If it turns out you like it, and want to install, I'd suggest Fedora. RedHat recently stopped officially supporting their free-download version. It's been taken over by the community, and is now called the Fedora Project. It's not like RH totally abandoned it, though... There are quite a few RH developers working on the project.