A GNU/Linux distro or distribution is a full-fledged GNU/Linux operating system packaged with all the software necessary to use it "out-of-the-box", including a Desktop Environment and Package Manager. Put together by committed groups of people, distros are usually branded with their own distinct name, logo and artwork.
One of the significant advantages of this development model is that if you ever get tired of one GNU/Linux distro, or you simply disagree with development changes, you can change distros without losing your applications and data. This is not the case with operating systems like Microsoft Windows and OS X. In fact, "forks" happen all the time, where one distro splits off into different distros because of such differences.
While some people prefer to build their own GNU/Linux distro from scratch, most people choose one of the many easy-to-use distros available. There are hundreds of active GNU/Linux distros.
Don't get overwhelmed by all the choices, though. Now that you know what a distro is, take a look at the following page for a short list of the most actively-developed and user-friendly distros today: Choosing a Distro