How to use new type ?

Hello ,

I've read a lot of documentation and i've applied what i've understood
and i've tried what i don't understand !

I've read that when i create a new table (test) , a new type test is
available .

So i can do : create table Mytable ( id int4 , tt test ) ;

But i've read something who said that before creating new type i'd to
create function to manipulate it ????

Is anybody can explain me ? I'm a newbie and my english is not very good
: i'm not lucky !

thanks a lot