In case of futures, a buyer of a contract is said to be “long position holder” and a seller is “Short position holder”. In the case of futures, to avoid the risk of defaulting contract involves both parties lodging a certain percentage margin of value of contract with a mutually trusted third party. Generally, in gold futures trading, margin varies between 2%-20% depending on the volatility of gold in spot market.
As an example, let's say an initial margin amount of $3,700 allows an investor to enter into a futures contract for 1,000 barrels of oil valued at $45,000—with oil priced at $45 per barrel. If the price of oil is trading at $60 at the contract's expiry, the investor has a $15 gain or a $15,000 profit. The trades would settle through the investor's brokerage account crediting the net difference of the two contracts. Most futures contracts will be cash settled, but some contracts will settle with the delivery of the underlying asset to a centralized processing warehouse.
The fee you are paying to buy the call option is called the premium (it's essentially the cost of buying the contract which will allow you to eventually buy the stock or security). In this sense, the premium of the call option is sort of like a down-payment like you would place on a house or car. When purchasing a call option, you agree with the seller on a strike price and are given the option to buy the security at a predetermined price (which doesn't change until the contract expires). 
A call option is a contract that gives the investor the right to buy a certain amount of shares (typically 100 per contract) of a certain security or commodity at a specified price over a certain amount of time. For example, a call option would allow a trader to buy a certain amount of shares of either stocks, bonds, or even other instruments like ETFs or indexes at a future time (by the expiration of the contract). 
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