A combo box is a combination of a list box and a text box.
Combo boxes are abundant through out the web and in desktop software.
Pros: Good for allowing a user to have a list of choices while requiring minimal space when not in use.
Problem: Becomes unwieldy and hard for a user if the list of choices is too long.
Potential Solution: Autocomplete Combo Boxes, using text matching on typed user input to narrow the list of possible choices and enable easier selection.
(This description was written by instanceOfTom.)