By Rabbi Julian Sinclair, June 30, 2011
Batmitzvah is the coming of age ceremony for Jewish girls of 12. From that age they are considered morally responsible for their actions and legally obligated to fulfil mitzvot. So far, no different from boys (except for the age).
As to the content of the ceremony, that is much less clear (at least in Orthodox communities, most of which do not have women reading from the Torah).