Describes a simple yes/no value. Unless the field has the required property set, it can actually have a third setting of unset. This can be useful as it distinguishes between a "False" and a "don't know/haven't filled this in yet". For example when asking if a paper is refereed, this is a boolean field. However it's not required as often someone isn't sure, and does not want to click "no" when that's not neccisarily(sp?) true.
|input_style||checkbox||radio|checkbox). This controls how the field is rendered in input forms.
Checkbox gives just a single HTML checkbox which can be ticked or not. It looks a bit rubbish, and does not provide a way to say