A "regular" view is a virtual table representing the result of a database query. Each time the view is accessed, the RDBMS must run the query to produce the result set for the view.
A materialized view is similar to regular view, except that the results are stored into an actual database table, not a virtual one. The result set is effectively cached for a period of time. When the underlying data changes the view becomes stale. Because of this, materialized views must be frequently "refreshed" to bring them up-to-date.
By caching result set data into a real table, the data can be accessed much more quickly, but at the cost of some data being potentially out-of-date. It is most useful in data warehousing scenarios, where frequent queries of the actual base tables can be extremely expensive. Since the materialized view is stored as a real table, you can build indexes on any combination of columns of the view, enabling drastic speedups in response time for queries that access the view.