QGIS has a bunch of mailing lists. See Mailinglists for the different available lists.
If you are going to ask questions please read this: How to ask a QGIS question?.
Nabble (http://nabble.com) keeps a history of a lot of mailinglists.
If you go to the osgeo section of it: http://osgeo-org.1560.x6.nabble.com/ you see there is a QGIS section also, where you can do a search over all QGIS lists, or refine it to eg only the users list.
QGIS does not have a forum. Please either search in StackExchange (see below), or via http://nabble.com in the history of our Mailinglists (see above)
A lot of the development buzz will be spoken on IRC.
There is a #qgis channel on freenode.net
You can also use a web interface: http://webchat.freenode.net/?channels=#qgis
TODO: write something about the IRC ethiquette?
If you want to file bugs or issues, you need an osgeo ID. You can create an account here: https://www2.osgeo.org/cgi-bin/ldap_create_user.py
More info to be found here: http://hub.qgis.org/wiki/quantum-gis/Bugreports
All issues can be browsed here: http://hub.qgis.org/projects/quantum-gis/issues