What is antifa, the far-left group tied to violent protests?

Antifa, the far-left militant movement that calls itself "antifascist," has no defined organizational hierarchy or membership process. The collection of autonomous antifa groups in mostly left-wing cities sees itself as a descendant of the European anti-Nazi movements, and generally agree that the best way to combat ideas they find odious is not through speech or debate but by direct action and physical confrontation.