Securities Transaction Tax (STT) is a way for the Government to earn revenue out of trades that take place on the Secondary markets. It was introduced as a levy in the year 2004 by the then Finance minister P.Chidambaram. It is quiet often seen by traders/investors as a deterrent to trade/invest in the secondary markets and the rates have reduced since the time it was introduced.
You’ll get the rates of STT in the link posted by Nithin below.