When DML changes are made to master table data, Oracle Database stores rows describing those changes in the materialized view log and then uses the materialized view log to refresh materialized views based on the master table. This process is called incremental or fast refresh. Without a materialized view log, Oracle Database must reexecute the materialized view query to refresh the materialized view. This process is called a complete refresh. Usually, a fast refresh takes less time than a complete refresh.

External Links:

SQL Snippets: Materialized Views - Materialized View Logs

Reference:

Oracle® Database SQL Reference 10g Release 1 (10.1) Part Number B10759-01
CREATE MATERIALIZED VIEW LOG