in list box we can add values but in combo box we can't ie 
in list box values added is not fixed but in combobox it is 
fix

combo box is having the features of listbox and textbox and 
in combo box we can see one item at a time but in list box 
more than one we can see depends on size.In listbox if 
number of items are increased then its size then one 
vertical scroll bar will be added to listbox intererly.

List box is select two or more selection but
combo box is selected by one.