Returning multiple rows in 8.4

I have users table such as
create table users(
user_id bigserial,
user_name varchar(50),
user_surname varchar(100)
);

how can I write a function to return multiple rows? ( in plpgsql )

what is the return value of the function? records?

and how can i run it without running as select my_func() as (int8,
varchar) ???
is there simple way to do this?

thanks