What is a Bat Mitzvah?
On reaching her 12th birthday every Jewish girl is able to celebrate her coming of age or "Bat Mitzvah" (sometimes known as Bat-Chayil) . The term refers to a new status of being able to fulfill the Mitzvot (G-D's commandment) as prescribed in the Torah.

How is it celebrated ?
Orthodox Bat-Mitvah ceremonies will normally be marked by a religious discourse given by the Bat-Mitvah girl at the Seudah (party).
Conservative or Reform Bat-Mitvahs may more closely resemble the Bar Mitzvah ceremony (see about Bar Mitzvah) and may involve the girl reading from the Torah during the Sabbath morning prayers.