The MySQL Performance Schema is a feature for monitoring MySQL Server execution at a low level. The Performance Schema is available as of MySQL 5.5.3.
From MySQL Manual there are some of the features of the performance_schema database.
The Performance Schema provides a way to inspect internal execution of
the server at runtime. It is implemented using the PERFORMANCE_SCHEMA
storage engine and the performance_schema database. The Performance
Schema focuses primarily on performance data. This differs from
INFORMATION_SCHEMA, which serves for inspection of metadata.
The Performance Schema monitors server events. An “event” is anything
the server does that takes time and has been instrumented so that
timing information can be collected. In general, an event could be a
function call, a wait for the operating system, a stage of an SQL
statement execution such as parsing or sorting, or an entire statement
or group of statements. Currently, event collection provides access to
information about synchronization calls (such as for mutexes) file and
table I/O, table locks, and so forth for the server and for several
Performance Schema events are distinct from events written to the
server's binary log (which describe data modifications) and Event
Scheduler events (which are a type of stored program).
For detailed information look this Link Performance Schema.