Functions

Dear PG members,I want to know that does begin and end in plpgsql are
related to begin transactions and commit? if yes/no how? If I start to
execute a function of multiple statements does PG commits each transaction
within a function, or whole function as a transaction.
Also if first statement makes inserts/updates a row, does it automatically
available to that particular function execution only or to any other
instances of same function?
Please give the details.

In my system, function are extensively used and cab called by different
clients as ODBC, JDBC and others.

Thanks for help,

CPK