Indexed views are schema-bound view objects in sql-server that have index and statistics dedicated to the view itself (as opposed to just on the underlying objects).
They can be very useful if applied correctly, but have a number of limitations. See this whitepaper from MS for an overview for SS2008.
The indexes can also be used even if the view is not referenced directly, which can be very powerful. They are most commonly used to materialize the results of aggregations on data that is relatively static.