Shani is the lord of the planet Saturn (one of the nine planets as per Hindu astrology), and is worshipped on Saturday, which literally means Saturn Day. The word Shani, also known as Shanaishchar, literally means the one who moves slowly and considering that Saturn takes about thirty years to orbit around the Sun, the name is an apt one for the lord of the planet.
Shani is the son of Surya, the Sun God, and Chhaya. Shani is also the brother of Yama and is considered the god of justice. Some consider Shani as the incarnation of Lord Vishnu to distribute the results of good and bad deeds of people.