When you book a table on OpenTable.com, your reservation is instantly recorded in the restaurant's computerized reservation book - the same one the restaurant's host uses to track all reservations. (It's the black terminal you may see when you arrive at the restaurant.)
Unlike other "online" reservation services which rely on faxes or table allocations, OpenTable has live Internet connections into the restaurants themselves. So restaurant availability information is up-to-date, and your reservations are confirmed.