stored proc vs sql query string

i have heard somewhere that writing a stored procedure, is much better than firing a sql query(such as select * from table_name) onto the database.
is it true and if yes how?

also i want to know that is the performnance in java slower as compared to cpp, given that the same things is being done.

i m using postgresql 8.0.0

thanks,
regards
Surabhi Ahuja