Lev Davidovich Bronstein, best known to history as Leon Trotsky, was murdered today, 70 years ago in Mexico City by Catalan Stalinist Ramón Mercader, who infamously used a pickaxe for the crime (and almost failed).
Trotsky led, together with Lenin and other bolsheviks, the Russian Revolution which, for the first time in history, brought a party of the Working Class to power. He was particularly relevant in organizing the Red Army, which became able to withstand not just the reactionary White forces but also the invading armies of Western powers that landed at several locations at the Russian coasts in support of the reaction.
It was also responsible however for the eventual destruction of the Anarchist Free Territory in Eastern Ukraine, one of the two almost successful Anarchist large-scale socio-political experiences in the 20th century (the other was the revolutionary zone in Catalonia and Eastern Aragon during the Spanish Civil War).
But surely Trotsky is best remembered as the main leader of the genuine Revolutionary Bolshevik faction after the death of Lenin, which was eventually defeated by the nationalist and fascistoid tendency lead by Stalin. As such, Trotsky was expelled from the party and exiled. Trotsky first had to go to the remote Alma Ata (now Almaty in Kazakhstan), then to Turkey, France, Norway and eventually Mexico, after European authorities decided they did not want such a revolutionary leader in their territories.
In Mexico he was hosted by the famous couple of revolutionary artists Diego Rivera and Frida Kahlo. Soon after breaking with Rivera, he moved to his final residence a few blocs away, where the infamous Mercader found and killed him.
As disillusion grew on Stalin's authoritarian and criminal tendencies, Trotsky became the main reference for genuine Leninism, which was often also called Trotskyism. The current was formalized in 1930 as the Fourth International. This current however has been often marred by schisms and sectarianism, although has also inspired many genuine revolutionary leaders such as Che Guevara.
Crucial concepts in Trotskyism are that the revolution should be permanent and international, that Stalinist USSR was not a genuine but a deformed or degenerated workers' state. Nowadays some Trotskyist sectors favor the creation of a new Fifth International, idea now promoted by Venezuelan President Hugo Chávez.
For further reading on Trotsky and Trotskyism, you may find useful Trotsky.net and the commemorative series at In Defense of Marxism.