Timeline

u-series (14.10) Future
Successor to Ubuntu Trusty (14.04)
Milestones:

The Ubuntu release that will be delivered in October 2014, designated 14.10.

Hardy (8.04) Obsolete
Successor to Ubuntu Gutsy (7.10)
Milestones: hardy-updates, ubuntu-8.04.4, ubuntu-8.04.3, ubuntu-8.04.2, ubuntu-8.04.1, ubuntu-8.04, ubuntu-8.04-beta, hardy-alpha-6, hardy-alpha-5, hardy-alpha-4, hardy-alpha-3, hardy-alpha-2, hardy-alpha-1

Hardy is the second Long Term Support ("LTS") release of Ubuntu, with a strong emphasis on supportability for long term deployments on both the desktop and the server.

Gutsy (7.10) Obsolete
Successor to Ubuntu Feisty (7.04)
Milestones: gutsy-updates, ubuntu-7.10, obsolete-milestone, ubuntu-7.10-rc, ubuntu-7.10-beta, tribe-6, tribe-5, tribe-4, tribe-3, tribe-2, tribe-1

Gutsy introduces a number of interesting desktop features, like Compiz Fusion for 3D desktop effects, and instant printer configuration.

Feisty (7.04) Obsolete
Successor to Ubuntu Edgy (6.10)
Milestones: ubuntu-7.04, 7.04-beta, herd-5, herd-4, herd-3, herd-2

Ubuntu 7.04 (Feisty) was released in April 2007. It focuses on improved hardware support across the board and lays the foundations for aggressive adoption of emerging desktop technologies. Ubuntu 7.04 returned to the standard six-month schedule following the shortened catch-up cycle used for Ubuntu 6.10.

Edgy (6.10) Obsolete
Successor to Ubuntu Dapper (6.06)
Milestones: later, edgy-updates, ubuntu-6.10, ubuntu-6.10-beta

Ubuntu 6.10 (Edgy) was released in October 2006. In the wake of the release of Ubuntu 6.06 LTS, Edgy includes more aggressive new features which were not feasible for an LTS release.

Dapper (6.06) Obsolete
Successor to Ubuntu Breezy (5.10)
Milestones: dapper-updates, ubuntu-6.06.2, ubuntu-6.06

Ubuntu 6.06 LTS (The Dapper Drake Release) is the first long-term-supported release of Ubuntu. Dapper features a GUI installer optimised for simplicity and speed and official support for a new server architecture: SPARC.

Breezy (5.10) Obsolete
Successor to Ubuntu Hoary (5.04)
Milestones: ubuntu-5.10

Breezy is the third installment of Ubuntu, released in October 2005.

Hoary (5.04) Obsolete
Successor to Ubuntu Warty (4.10)
Milestones: ubuntu-5.04, ubuntu-5.04-preview

Hoary is the second release of Ubuntu, pushed to mirrors in April 2005.

Warty (4.10) Obsolete
Milestones: ubuntu-4.10

Warty is the first release of Ubuntu. Warty was released in October 2004.