dropdown menu

Hi Cmoi,
Welcome to MVD.
What you are looking at isn't actually a drop down menu, it's a drop down combobox.
It contains a list of valid values from which an item can be chosen when adding or editing a record.
Have a look at the attachement which is a much simpler example than the 'library' application you've looked at. 
It has two tables ('people' and 'countries').  The 'countries' table contains a list of countries, one of which can be chosen by using the drop down combobox when adding or editing a record in the 'people' table.  This is made possible by the relationship that has been specified between the 'people' and 'countries' tables.
Derek.