I am confused as to which option (Listbox or Combobox) to use when.

I am designing a userform to input information pertaining to a flight plan.
Instances where I would use either listbox or combobox would be to:

1) list various airports that are listed on worksheet "Airports"
2) list different planes a pilot is certified to fly

I do not want the user to be able to add to the list. The only way
something can be added is if information about the plane or airport is
entered on a spreadsheet and the combobox/listbox will pick up the new info
automatically.

Can you please explain what the defining difference is between the two
different options so I can decide what to use?

Thanks,
Les