The INFORMATION_SCHEMA views allow you to retrieve metadata about the objects within a database. These views can be found in the master database under Views / System Views and be called from any database in your SQL Server instance. The reason these were developed was so that they are standard across all database platforms. In SQL 2005 and SQL 2008 these Information Schema views comply with the ISO standard.
Following is a list of each of the views that exist.
These views can be used from any of your databases. So if you want to gather data about Tables from the AdventureWorks database you would issue the following in that database.
SELECT * FROM INFORMATION_SCHEMA.TABLES