In general:
1) Rwanda was originally a Colonial territory, most notably occupied by Belgium. For their own interests, the Belgians artificially created a social division between the Hutu and Tutsi people.
2) While there are no extreme ethnic differences between these two groups, Rwanda’s ruling imperial powers attempted to create such differences, placing the Tutsi minority above the Hutu in social strata.
3) The division of social classes and clear favoritism generated friction between the two groups culminating in the events of the 1994 genocide.
I wrote a paper for Professor Daniel Levine in my freshman year regarding UN operations during the genocide.