What makes them a "bruiser"? By that I mean which stat defines them as a bruiser or is it something else that makes them a bruiser?
Tanky dps. Bruisers generally also have pretty strong lane sustain that allows them to farm. They're steady and reliable DPS for your team, not high or accurate enough to be a classic carry but enough of a threat to warrant focusing.
Yeah. They're pretty beefy- Perhaps even enough to offtank. They can also dish out TONS OF DAMAGE (As Phreak would say), but not enough to count as a true Carry a la Tryndamere, Yi, etc.
A bruiser is a strong sustain hero that can deal damage consistently.