Timeline

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

Debian Experimental

Sid (98) Active Development
Successor to Debian Lenny (5.0)
Milestones:

The current development snapshot is named sid. Untested candidate packages for future releases.

Buster (10.0) Future
Successor to Debian Experimental (99)
Milestones:

This release started as a copy of stretch, 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.

Stretch (9.0) Current Stable Release
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).

Wheezy (7.0) Supported
Successor to Debian Squeeze (6.0)
Milestones:

Debian 7.0 was released May 4th, 2013. The release included many major changes, described in our press release (https://www.debian.org/News/2013/20130504) and the Release Notes (https://www.debian.org/releases/wheezy/releasenotes).

Squeeze (6.0) Obsolete
Successor to Debian Lenny (5.0)
Milestones:

Debian 6.0.0 was released February 6th, 2011. The release included many major changes, described in our press release (http://debian.org/News/2011/20110205a) and the Release Notes (http://debian.org/releases/squeeze/releasenotes).

Lenny (5.0) Obsolete
Milestones:

Debian GNU/Linux 5.0.0 was released February 14th, 2009. The release included many major changes, described in our press release (http://www.debian.org/News/2009/20090214) and the Release Notes (http://www.debian.org/releases/lenny/releasenotes).