Linux Mint Debian Edition is a Debian based distribution maintained by the Linux Mint team. It is available in Cinnamon and MATE flavours. Betsy is based on Debian Jessie which is currently under the “testing” Debian branch.
Currently it’s not trivial to upgrade from earlier LMDE releases, but an upgrade path will be provided after the stable release. It is likely to be done via command-line.
For more information and download links check the official announcements: