From the release announcement: "After 26 months of development the Debian project is proud to present its new stable version 9 (code name "Stretch"), which will be supported for the next 5 years thanks to the combined work of the Debian Security team  and of the Debian Long Term Support  team.". Release Notes and Installation Guide.
Some new things:
The UEFI ("Unified Extensible Firmware Interface") support first introduced in "Wheezy" continues to be greatly improved in "Stretch", and also supports installing on 32-bit UEFI firmware with a 64-bit kernel. The Debian live images now include support for UEFI booting as a new feature, too.
(first seen on LWN.net)