You can think of a distribution as a different flavor or version of Linux. Much like Microsoft has Windows 3.1, Windows 95, Windows 98, Windows XP, and Vista; Linux has Fedora, Ubuntu, Mandriva, SuSE, PCLinuxOS, gOS, and many more. Some of these distributions have major differences and some of them have minor differences. But each distribution is built upon the Linux kernel.
The most user-friendly distributions are Ubuntu, PCLinuxOS, and Mepis. The more difficult distributions are Gentoo, Fedora, and Slackware.