何时使用USE开始T-SQL查询？

I'm a T-SQl and database newbie, and a little confused. In the T-SQL book I'm reading it says that a USE statement is written to set the database context of the session. Does that mean that there can be more than one database in an instance of SQL-server and the USE statement tells SQL server which database the query will be on?I'm a T-SQl and database newbie, and a little c