What is the usage of performance_schema in MySQL 5.5?
All of your answers can be found by reading the documentation:


What is the usage of this new schema..?



  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.



What are the advantages of using this schema..?



  The Performance Schema is a tool to help a DBA do performance tuning by taking real measurements instead of “wild guesses.” source



Is there any overhead by enabling of this schema..?



  Server monitoring occurs continuously and unobtrusively with very little overhead. Activating the Performance Schema does not make the server unusable.



How can we monitor using this schema..?


This page gives a basic overview of how to use the performance schema