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; and as a mandatory system extension as of TYPO3 6.0.

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. Meetings dates are announced on Twitter by @extbase. The notes for the next meeting can be found here https://notes.typo3.org/p/Extbase_Teammeeting_next

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

Anja Leichsenring (maddy2101)
Felix Oertel (foertel)

Member

Alexander Schnitzler (alex_schnitzler)
Daniel Hürtgen (higidi)
Marc Bastian Heinrichs (basheinr)
Markus Günther (m.u.guenther)
Stefan Froemken (froemken)
Stefan Neufeind (neufeind)
Tymoteusz Motylewski (tmotylewski)
Wouter Wolters (wouter90)

Bot

Gerrit Code Review (gerrit-review)
TYPO3 Release Team (typo3v4)

Honorary Member

Bastian Waidelich (bwaidelich)
Jochen Rau (jocrau)
Sebastian Kurfuerst (sebastian)

Scrum Master

Robert Weißgraeber (weissgraeber)