] Each index server indexes only part of the Web, as the whole Web
] will not fit on a single machine -- certainly not the type of machines
] that Google uses. Google's index of the Web is distributed across
] many machines, and the query gets sent to many of them -- Google
] calls each on a shard (of the Web). Each one works on its part of the
] problem.

How Google scales.

The magic that makes Google tick: ZDNet Australia: Insight: Software