[Koha-devel] what are biblio, biblioitems and items tables

I can't understand what means the 3 tables biblio biblioitems and items. A biblio contains the datas for a book we can have, or not have in our library. It contains the fact that such a document EXISTS. An item is a data to explain that we POSSESS a biblio, where, since when, at which costs... But what's the hell is a biblioitem ? Where is it used ? How could be a definition constraint with biblio/item be writen if mySQL supported them ? Thank's -- Paul