Here are a couple of posts from the MSDN archive. They had been taken out of general circulation, but have since been reintroduced to the library. Together they make up one of the most comprehensive reviews of developing with combo boxes and list boxes I've seen.
Programming Combo Box and List Box Controls in Microsoft Access, Part 1
Part one reviews the various options for supplying a list of values to a combo box or list box (all but one), including one I didn't know existed, the Field List setting. It also covers off how to find out what the user selected, including in a multi-select list box. It talks about using the NotInList event to make your lists dynamic. The only thing I can see missing is cascading combo boxes, which I cover off in these posts.
Programming Combo Box and List Box Controls in Microsoft Access, Part 2
Part two goes into using a function to provide values to the list. This one's a little more complex, but really not difficult, and provides functionality the other options don't.