System views are predefined Microsoft created views for extracting SQL Server metadata. System Views can be found under System Databases -> master -> Views -> System Views.
The first group of System Views belongs to the Information Schema set. INFORMATION_SCHEMA contains 20 different views. Most of the Information Schema view names are self-explanatory. For example INFORMATION_SCHEMA.TABLES returns a row for each table. INFORMATION_SCHEMA.COLUMNS returns a row for each column.
INFORMATION_SCHEMA is contained in each database and each INFORMATION_SCHEMA view contains meta data for all data objects stored in that particular database. For example one can retirieve all the constraint information on a particular table etc.