Usually a small box that can be toggled either "on" of "off." When clicked, an "x" or checkmark appears signifying an "on" option.
Within WebCT, a selection box that usually indicates more than one option can be selected. For instance, clicking multiple files within Manage Files will allow you to zip those that are selected. In a multiple choice question it means you may select more than one answer.
In the OPEN LOOK GUI, a nonexclusive setting that shows a checkmark in a square box when the setting is chosen.