In MySQL version 5.02 and above there is an additional database called information_schema. This database provides access to the database metadata. This includes the name of a database or a table, the data type of a column, access privileges, etc.
information_schema is the information database where the information about all the other databases is kept. It is a built-in virtual database with the sole purpose of providing information about the database system itself. The MySQL server automatically populates the tables in the information_schema.
The important thing to remember about the information_schema database is that you can query it, but you cannot change its structure or modify its data.