The widely respected authority on Linux distros is
DistroWatch. They keep a running list by popularity over on the right side of the page, where currently the most popular distros are Ubuntu, PCLinuxOS, openSUSE, Mint and Fedora. They also have a summary page for each of the hundreds they track, which includes download links.
Personally, my favorite is
Mandriva, which I've been using daily for 6 years now. But I also have
PCLinuxOS (also called PCLOS) and
Fedora installed on other computers and I like them too.
In my eyes, the main thing that separates the various distros is the packaging system. There are three groups: RPM, Debian and other.
RPM distros include Fedora, Mandriva, PCLinuxOS, CentOS and Scientific Linux.
Debian distros include Debian, Ubuntu (plus Kubuntu, Xubuntu, etc), Mint and MEPIS.
Other distros include Gentoo, Sabayon, Arch and Vector.
But another very important thing you must choose is which desktop environment or window manager to install. Most distros will allow multiple choices, but most also have a default. The two big choices are KDE and Gnome. Smaller choices include XFCE and IceWM. (I prefer IceWM.)
In general, for a beginner it's probably best to just allow the installation software to install the default environment. But I mention it because the desktop environment often makes a bigger difference in appearance and look-and-feel than the distro does.