Microsoft SQL Server 2005 uses reserved keywords for defining, manipulating, and accessing databases. Reserved keywords are part of the grammar of the Transact-SQL language that is used by SQL Server to parse and understand Transact-SQL statements and batches. It is not legal to include the reserved keywords in a Transact-SQL statement in any location except that defined by SQL Server. No objects in the database should be given a name that matches a reserved keyword. Although it is syntactically possible to use SQL Server reserved keywords as identifiers and object names in Transact-SQL scripts, you can do this only by using delimited identifiers.
Please use Microsoft MSDN BOL to see all the Reserved Keywords. The text above has been referenced from MSDN BOL.
Reference : Pinal Dave (http://blog.SQLAuthority.com)