Simchat Torah (שמחת תורה) means "rejoicing with the Torah". It actually refers to a special ceremony which takes place on the holiday of Shemini Atzeret. This holiday immediately follows the conclusion of the holiday of Sukkot. In Israel, Shemini Atzeret is one day long and includes the celebration of Simchat Torah. Outside Israel, Shemini Atzeret is two days long and Simchat Torah is observed on the second day, which is often referred to by the name of the ceremony.
The last portion of the Torah is read, completing the annual cycle, followed by the first chapter of Genesis. Services are especially joyous, and all attendees, young and old, are involved.
(Adapted from Wikipedia)