What are the illuminated manuscripts? how were they made? what were their original purpose? what are their main features?

        An illuminated manuscript is a manuscript in which the text is supplemented with such decoration as initials, borders (marginalia) and miniature illustrations.
         It is made from animal skin, most usually the skin of a sheep or cow. Goatskin was used too  and also deer or pigskin .
         Illuminated Manuscripts were used by priests and monks for ritual purposes.
I know all three of these questions but I cannot figure out the last question. But I hope this helps.