Prior to the delivery date, contracts are reversed or closed out. This is done by the clearinghouse. Thus, the owner of a futures contract to buy silver can close out their position by entering into a futures contract to sell silver. The clearinghouse will net out the position to zero at the end of the day. Marking to market is also done by the clearinghouse at the end of each trading day using the settlement price.
Futures also differ in that they have a two-sided potential for a gain or loss.
A futures contract can be thought of two parties simultaneously owning the same security or commodity. If the underlying stock or commodity increases in value, gains will accrue to one party at the expense of the other. Vice versa if the value declines. Thus, whatever one gains, the other will loss. Essentially, a futures contract can be thought of as owning a call option and selling or writing a put option.