All related. Technically, bruxism, is the habitual, involuntary grinding or clenching of the teeth, usually unconsciously, and most often occurring during sleep, and sometimes associated with stress. While grinding of the teeth is different from clenching your teeth, they usually occur together for the most part.
A matter of degree. Bruxism refers to tooth grinding (but also encompasses tooth clenching). It is the habit that produces extra tooth to tooth contact. With tooth grinding the result can be tooth wear, chipping, breaking or increased mobility (looseness). Tooth clenching can result in tooth fractures. Additionally, bruxism can result in TMJ pain or myofacial (jaw muscle) pain.
Same thing. Same thing.