Middle-class champagne socialist/communist/anarchist white boys who don't like nationalists or fascists. They consider themselves to be rebelling against the establishment, whilst upholding all of its ultra-politically correct views.
Antifa only dislike racism when its carried out by whites, and do not have the bottle to stand up against anti-white racism; leading to many people on the right to refer to them as 'traitors'. I'd rather just call them morons.
Most are teenagers and university students who grow out of the fad when they start paying taxes.