Debian has been around for what feels like forever and it provides the base for hundreds of other distributions including Ubuntu and Linux Mint.
Debian contains a set of repositories with an incredibly large number of applications available for users to install.
The versions of Debian available on their site only install free software and there are no third party or proprietary products included by default.
For new users, setting up Debian can be somewhat of a challenge compared to Ubuntu or Linux Mint.
The choice of which version of Debian to run is also quite tricky and depends on the person who will be using it. If you want the latest stuff today then you can install the unstable branch which has all the latest products but they may or may not work for you. At the other end of the scale you can choose the stable branch which has older versions of software that are pretty much guaranteed to work.
Debian is like Linux Lego. It is great for people who want to start from a base installation and build something from the ground up. It may not be suitable for people who have limited computer skills and it requires more of a learning curve than Linux Mint or Ubuntu.