Definition of a Future

Before starting trading with any kind of future system it's important to know what a future exactly is.

A future is an agreement to buy or sell a commodity or financial asset at a certain time in the future. 

Most future contracts are subjected to a cash settlement procedure. Please make sure to verify the contract specifications at the exchange where the future is listed. For E-mini futures take a look at the website of the CME.

Leverage

Whenever trading futures you have to realize that you're working with a lot of leverage.

Example:

Suppose you buy one E-mini Nasdaq100 future right now. A future is an agreement to buy or sell in the future, so you don't have to pay any money at this moment. 

Each point in the Nasdaq100 E-mini future represents 20 dollar. From the moment you have bought the future till you sell it ( or the future expires) you have to pay 20 dollar for each point the Nasdaq100 future goes down and you will receive 20 dollar for each point the Nasdaq100 future will rise.

Margin

Of course your broker will ask you to deposit a margin amount. The margin for a Nasdaq100 future (depending on broker) is approximately 4.000 US dollar. So instead of paying 20* 1500 (price of one cash index)= 30.000 dollar you have to deposit 4.000 dollar.