List boxes can act as a set of radio buttons (allowing people to select exactly one choice from a set of mutually exclusive options) or as a set of checkboxes (allowing people to select any number of choices from a list of options). List boxes can be configured to show more options than a drop-down menu while still taking up less screen real estate than a list of radio buttons or checkboxes. Despite these advantages, the dual nature of list boxes (mutually exclusive single selection or multiple selection) tends to cause problems for many people. As a result, list boxes are rarely used in Web forms.
On Functioning Form, I discuss an alternative solution for selecting multiple options within a form: Alternate Select Multiple.