creating type

Hi there,
i am trying to make a new type for my application. Which happen to be a structure of this kind: <text,timestamp>.
It is said that on creating types, function which convert external form of the type to internal form, and vice versa,
shouldbe created first. I don't get the picture. Can anybody explain to me, especially about the internal form of the
type? What is external form, and what is internal form ? And perhaps, anybody can give an example script of creating
type(s)including one of the INPUT/OUTPUT function? so that i can get the picture better. 

thank you very much,
regards,
rifki
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+   open mind, open heart, act !  +
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