[ADMIN] 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