A Linux Distribution is kind of like a flavor of Linux, just like soda pop have different colors and flavors they all have one thing in common, besides not being to healthy for you, the carbonated water and sugar. Some of the major differences in distros are
1. commercial or non-commercial;
2. designed for enterprise or for home usage;
3. designed for servers, desktops, or embedded devices;
4. targeted at regular users or power users;
5. general purpose or highly specialized toward specific machine functionalities, for example firewalls, network routers, and computer clusters;
6. designed and even certified for specific hardware and computer architectures;
7. targeted at specific user groups, for example through language internationalization and localization, or through inclusion of many music production or scientific computing packages.

Some of the most common distros available today are: Slackware, Debian, Ubuntu, Red Hat, CentOS, Mandriva, openSUSE, Gentoo, Knoppix (live CD only), Linspire. Of course there are ways to track what is new with every distro along with seeing exactly how popular each distro is by going to this site. Lost don't know what distro is for you, don't fear their is a good site that has a quiz which can give you an idea of a couple of distros you should look into, that quiz can be found here.