Ernesto "Che" Guevara was an Argentinian Marxist Revolutionary. He was one of Fidel Castro's key subordinates in the Cuban Revolution, where he gained a reputation for guerilla warfare and executing people. After the revolution succeeded there instead of staying in Cuba trying to make socialism work he went and tried to export revolution elsewhere.
Good and bad
(..) he [Che] was much more enamored of other people’s deaths. In April 1967, speaking from experience, he summed up his homicidal idea of justice in his “Message to the Tricontinental”: “hatred as an element of struggle; unbending hatred for the enemy, which pushes a human being beyond his natural limitations, making him into an effective, violent, selective, and cold-blooded killing machine.” 
Conservatives, Fascists and the Mafia hate him for his role in kicking their Mafia puppet regime out of Cuba. Liberals share his hatred of the evil Conservative/military dictatorships that ruled Latin America in the interests of the rich and the military castes, and rather wish he'd been fighting for a better cause. How different would World History have been if Che had been a Liberal or Social Democrat or Progressive Democratic Socialist.
Che Guevara is most famous for his book Guerrilla Warfare, which documents the art of exactly that. He also wrote The Motorcycle Diaries, which is a set of diaries written on his 5,000 kilometer trip across South and North America. In it, he throws down on imperialism and gives North America a verbal bitch slap.
He is best known for a particularly iconic overly commercialised photo and for having a really cool beret and a kick-ass beard. He was executed in Bolivia in 1967, with the help of the CIA after failing to overthrow that Government.