Timeline

Experimental (99) Experimental
Successor to Debian Lenny (5.0)
Milestones:

Debian Experimental

Bookworm (12) Future
Successor to Debian Experimental (99)
Milestones:

This release started as a copy of bullseye, and is currently in a state called "testing". That means that things should not break as badly as in unstable or experimental distributions, because packages are allowed to enter this distribution only after a certain period of time has passed, and when they don't have any release-critical bugs filed against them.

Bullseye (11) Current Stable Release
Successor to Debian Experimental (99)
Milestones:

Debian 11.0 was released on August 14th, 2021. The release included many major changes, described in our press release (https://www.debian.org/News/2021/20210814) and the Release Notes (https://www.debian.org/releases/bullseye/releasenotes).

Buster (10) Supported
Successor to Debian Experimental (99)
Milestones:

Debian 10.0 was released July 6th, 2019. The release included many major changes, described in our press release (https://www.debian.org/News/2019/20190706) and the Release Notes (https://www.debian.org/releases/buster/releasenotes).

Stretch (9.0) Supported
Successor to Debian Experimental (99)
Milestones:

Debian 9.0 was released June 17th, 2017. The release included many major changes, described in our press release (https://www.debian.org/News/2017/20170617.en.html) and the Release Notes (https://www.debian.org/releases/stretch/releasenotes).

Jessie (8.0) Supported
Successor to Debian Experimental (99)
Milestones:

Debian 8.0 was released April 25th, 2015. The release included many major changes, described in our press release (https://www.debian.org/News/2015/20150426) and the Release Notes (https://www.debian.org/releases/jessie/releasenotes).