A data entry element that corresponds to fields with only two valid values, usually enabled (selected) and disabled (clear); the checkmarks toggle on and off as the check boxes are clicked. A click in an empty box selects that option; selecting a check box that already has a checkmark disables that option.
Appears visually as a square box the user can click on to mark or unmark an option. It usually appears in a form. A screen reader will also allow you to mark/unmark a checkbox.
The little square boxes that appear to the left of some fields. They are used as on-off switches. Where an X appears in the box the associated condition applies, when the box is empty the associated condition does not apply. In text mode check boxes are usually shown as [ ].