Hotel Rwanda did a good job of introducing it. Basically there were several racial groups in Rwanda for centuries, most prominently the ruling Tutsis and the much more common Hutu. When the Belgians took over after World War I, they figured that a divided population would be easier to rule, so they took advantage of the elite, and supposedly more European, Tutsi class. Hutus were systematically oppressed, got angry, and rebelled in 1959. They got a new democratic government elected by the majority Hutu population, so it largely favored the Hutus for the next three decades. There was a strong form of reverse discrimination against the Tutsis. For instance, only 9% of civil service and highly skilled professional jobs could be held by Tutsis, causing a lot of Tutsi's to be forced out of jobs and homes. Tensions were high, including a civil war as the Tutsi's try to come back and get equal rights. Finally, in 1994, the Hutu president's plane is shot down and BAM! Hutu's are pissed and start killing every Tutsi they can.

A couple of followup questions- now that 20 years has passed.

Has the Rwandan society changed significantly? Both in relation to ratios of Hs and Ts and in relation to violence in general?

What changes has the political system been subjected to from outside in response to the genocide? Did the world political system change in relation to this and was that used in other places?