In addition to what the other guy said, Mint is also more focused on desktop. A bunch of apps are pre-installed that one would expect on a desktop OS. Additionally, the default Mint UI, Cinnamon, feels very familiar to a Windows user. It has a start menu, task bar, tray, etc.

Debian is in the same family, and is more oriented for servers. It is super minimal out of the box, which is perfect when you want it to sit in the other room and perform specific tasks. However, you can install all the same programs, even the Cinnamon UI on Debian.

Really the difference is the out of box experience, but they are otherwise pretty similar.