It is a contract to buy or sell specified quantity of an underlying asset at a specified future date, at a specified price. These contracts are traded and settled on exchanges. The quantity (Lot size) and the Settlement date (Expiry date) is fixed in advance. In India, futures are cash settled. It is a leverage product as compared to cash market.

For example: In cash segment funds required for buying 250 shares of Axis Bank @ Rs. 1500 are Rs. 3,75,000 . If the price increase to Rs. 1600 you make a profit of Rs. 25,000 (250*100) in Cash market. 

Whereas in  Futures market, you pay a margin  to take a futures position. You pay only Rs. 75,000 (assuming 20% margin for 1 lot of Axis Bank = 20%*250*1500) to take a position of Rs. 3,75,000 and if price increases to 1600 you will make a profit of Rs.25,000.