What are stored procedures ? How is it useful ?

A stored procedure is a set of statements / commands which reside in the database. The stored procedure is pre-compiled and saves the database the effort of parsing and compiling sql statements everytime a query is run.
Each database has its own stored procedure language.
Before the advent of 3-tier / n-tier architecture, it was pretty common for stored procs to implement the business logic ( A lot of systems still do it).
The biggest advantage is of course speed. Also certain kind of data manipulations are not achieved in SQL. Stored procs provide a mechanism to do these manipulations. Stored procs are also useful when you want to do Batch updates / exports / houseKeeping kind of stuff on the db. The overhead of a JDBC Connection may be significant in these cases.