In List Box we have to choose one of the option from the 
drop down list 
In Combo just like radio button we have to choose any one

ListBox is used to select one item out of diffenrent we 
have.
But
ComboBox is used to enter an item at run time.

in ListBox u can select multiple items.
but in Combo box u can select only one item.

The listbox control is used to display a list of items to a 
user.
Combobox control is used to display a drop- down list of 
items . It is a combination of a text box in which a user 
enter an items,and a drop-downlist from which the user 
selects an item.

In listbox we can select more than one item.
but in case of combobox we dont have that chance,we have to 
select only one item from the list of items.

1) Combo Box:												
a. User should be able to enter text in it.												
b. Pressing the arrow should give list of options available
to the user. List can be scrollable but user should not be
able to type in.												
c. Pressing Ctrl-F4 should open the list box.												
d. Pressing a letter should bring the first item in the list
starting with the same letter.												
e. Items should be in alphabetical
order in any list.												
f. Selected item should be displayed on the list.												
g. There should be only one blank space in the
dropdown list.												
												
2) List Boxes:												
a. Should allow single select, either by mouse or
arrow keys.												
b. Pressing any letter should take you to the first element
starting with that letter												
c. If there are view/open button, double clicking on icon
should be mapped to these behaviour.												
d. Make sure that all the data can be seen using
scroll bar.