Linux Mint main edition is based on Ubuntu, just as Ubuntu is based on Debian (LMDE, or Linux Mint Debian Edition, is based directly off of Debian), but Clem and company make quite a few under-the-hood changes, include their own PPAs and proprietary codecs by default (as well as having the getdeb repository just waiting for a check mark to be enabled).
There are a lot of differences, but the biggest obvious difference to an end user (other than more media simply working out of the box due to the inclusion of more codecs by default) is the interface and its focus on user-friendliness and customability.
Although the other usual suspects are of course available, the main editions use either Cinnamon—developed in-house—and MATE, which Mint helps sponsor development for. MATE is a fork of the old, user-friendly Gnome 2, whereas Cinnamon, while resembling Gnome 2 in many respects, is the result of Clem and company asking users what they wanted in an interface, and then building it. (Exactly the opposite of what both Ubuntu and the Gnome 3 team did with their new interfaces.)
Webupd8 has a screenshot tour of the just-released Mint 15 here.