STT is a tax levied by the government on every purchase and sale of securities listed on the stock exchanges. 

STT varies for different products: 

Equity Delivery0.1% on Buy & Sell-side
Equity Intraday0.025% on Sell-side
Equity Futures0.01% on Sell-side
Equity Option0.05% on Sell-side (on premium)
Currency FuturesNo STT
Currency OptionsNo STT
Commodity Futures0.01% on sell side (Non-agri)
Commodity Options0.05% on sell-side