As a side-story of A Certain Magical Index, this focuses on Mikoto Misaka , an electromaster who is the third most powerful of the seven Level 5 espers in Academy City. Set before and during the events of the novels, it tells the adventures of Mikoto and her friends during that time period from their point of view.

The first 12 episodes follow the manga closely, with some changes and a few anime-original episodes and characters, while the later half of the series being a new original plot written by Kazuma Kamachi. Eight DVD compilations, each containing three episodes and a mini Toaru Majutsu no Index novel written by Kazuma Kamachi and art by Haimura Kiyotaka which features Kaori Kanzaki as the protagonist, were released since February 7, 2010. A five minute bonus episode was included with the Official Visual Book of the show released on July 24, 2010. An OVA episode was then released on October 29, 2010.

Source:Wikipedia