In futures, exchange will get the refundable margin from buyer and seller. The contract will be traded between buyer and seller on the strike price for the stocks. Once the contract is agreed, exchange will take care of mark to market on daily basis until the end of the contract.
In options, exchange will execute the trading of the premium between buyer and seller and once agreed, exchange will take care of daily leveling, but the buyer of the contract do not have any obligations.
Exchange plays the role of mediator in derivative instruments. In Futures the exchange collects the margin from both the buyer and seller, does the mark to market and on the expiry date makes the settlement for the deal happen. In options exchange settles the premium deal between the buyer and seller, does the mark to market on a daily basis and takes care of the final settlement