When a Jewish child reaches the age of majority (12 for girls, 13 for boys) that child then becomes responsible for him/herself under Jewish law; at this point a male child is said to become Bar mitzvah; a female child is said to become Bat mitzvah.
Previous to this age, all responsibility is with the parents. After this age the children are now privileged to participate in all areas of Jewish communal life, and they bear their own responsibility in the areas of Jewish ritual law, tradition, and ethics.
Since medieval times it has been traditional for a boy to celebrate becoming Bar Mitzvah. On the Shabbat after his 13th birthday, he reads from the Torah and Haftara, gives a d'var Torah (homily) and leads part of the prayer services. This is followed by a celebratory meal with family, friends, and members of the community. However, except in Italy, no similar ceremony evolved at that time for Jewish girls.
Traditionally, this celebration was limited to males. This is because women were not considered obliged to obey the same laws as men; since there was no obligation for adults, there was no issue of a change in a girl's status.
In the nineteenth and twentieth centuries, however, Jews of the Reform, Conservative, and Reconstructionist movements reconceived the notion of "obligation," as well as gender distinctions, within Judaism. Today most Jews celebrate a girl's attainment of becoming Bat Mitzvah in the same fashion as a boy's. Most Reform and Conservative synagogues have egalitarian participation in which women may read from the Torah and lead services. Conservative Judaism is pluralistic, and some synagogues are still concerned about the halakhic propriety of women reading the Torah portion to men (some believe that a woman's voice distracts men from fulfilling their obligation in a pious spirit); in such congregations girls read from the Haftara. Reform Judaism is entirely egalitarian! The majority of Orthodox Judaism rejects the idea that a woman can read from the Torah to men or that a woman may lead prayer services, and so has developed a less public way to mark this occasion.