Both types of contracts are binding and are typically settled for cash for the exchange in question upon expiry, although contracts can also be bought and sold before they expire. The forwards and futures markets can offer protection against risk when trading currencies. Usually, big international corporations use these markets in order to hedge against future exchange rate fluctuations, but speculators take part in these markets as well.
The broker basically resets the positions and provides either a credit or debit for the interest rate differential between the two currencies in the pairs being held. The trade carries on and the trader doesn't need to deliver or settle the transaction. When the trade is closed the trader realizes their profit or loss based on their original transaction price and the price they closed the trade at. The rollover credits or debits could either add to this gain or detract from it.
Imagine a trader who expects interest rates to rise in the U.S. compared to Australia while the exchange rate between the two currencies (AUD/USD) is .71 (it takes $.71 USD to buy $1.00 AUD). The trader believes higher interest rates in the U.S. will increase demand for USD, and therefore the AUD/USD exchange rate will fall because it will require fewer, stronger USD to buy an AUD.