Extbase is a backport of some features of FLOW3 to TYPO3 v4. You might want to start with the Documentation.
Extbase is shipped with TYPO3 as a system extension since version 4.3.

Getting started

At the moment the central place for Extbase/Fluid documentation is the wiki

Version sheme

- Extbase 1.1: included in TYPO3 4.3
- Extbase 1.2: included in TYPO3 4.4
- Extbase 1.3: included in TYPO3 4.5 LTS
- Extbase 1.4: included in TYPO3 4.6

From TYPO3 4.7, the Extbase version will be in sync with the core. Thus Extbase 4.7 will be shipped with TYPO3 4.7.

Extbase Team Meetings

The ETM was our attempt to increase the transparency and make collaboration easier. But as it appears at the moment, there is demand for that kind of meeting. After some ETMs with just one or two participants, we decided to suspend the meetings until further notice.

Here is a list of the Previous Meetings
And the issues we compile from the meetings: http://forge.typo3.org/projects/typo3v4-mvc/issues?query_id=199

Issue tracking

View all issues

Members

Leader

Felix Oertel (foertel)

Co-Leader

Sebastian Kurfuerst (sebastian)

Member

Bastian Waidelich (bwaidelich)
Christian Kuhn (lolli)
Christian Mueller (kitsunet)
Claus Due (claus@wildside.dk)
Gerrit Code Review (gerrit-review)
Jochen Rau (jocrau)
Marc Bastian Heinrichs (basheinr)
Patrick Lobacher (pschuster)
Sacha (st0rz)
Stefan Neufeind (neufeind)
Xavier Perseguers (xperseguers)

Pending Member

Daniel Lorenz (capsicumnet)

Bot

TYPO3 v4 Release Team (typo3v4)