The following databases exist for MySQL’s use. You may read (SELECT) them, but you must not write (INSERT/UPDATE/DELETE) the tables in them. (There are a few exceptions.)
mysql – repository for GRANT info and some other things.information_schema – The tables here are ‘virtual’ in the sense that they are actually manifested by in-memory structures. Their contents include the schema for all tables.performance_schema – ?? [please accept, then edit]