timeline

The "trunk" series represents the primary line of development rather than a stable release branch. This is sometimes also called MAIN or HEAD.

This is the "mitaka" series.

This is the "liberty" series.
See http://releases.openstack.org/liberty/index.html#neutron

This is the "kilo" series.

This is the "juno" series.

The juno series for the incubator

This is the "icehouse" series.

This is the "havana" series.

This is the "grizzly" series.

The future series tracks work that is necessary but will not land (or be completed) in the current development cycle.

This is the Folsom series of Quantum.

This is the "Essex" series which will become the 2012.1 release

Diablo development series (Quantum is mirroring the standard OpenStack naming scheme and release cycle)