Hutus (Bahutu) belong to the greater Bantu macro-ethnicity, that
migrated from Central Africa (Cameroon?) more than 1,000 years ago,
with the arrival of iron tech, colonizing both the Congo jungle and
most of Eastern and Southern Africa. They are about 85% of Rwanda and
Burundi peoples and they are basically peasants.
Tutsis (Batutsi) belong to the Nilotic macro-ethnicity that came from
somewhere near Sudan originally. They are a lot taller than Hutus and
they are basically cattle-herders. They are also spread in Uganda and
Ethiopia among other Eastern African countries.
Before the arrival of Bantus and Nilotics, the region was inhabited by Pigmy Twa peoples, now just a tiny minority.
I'm unsure about what roles both ethnicities played in the precolonial
states but a dominance of the Tutsi minority seems rather clear. This
aristocratic role was favored by colonial authorities in the typical
policy of divide et vince.
Yet, in Rwanda, the last Tutsi king fled in 1959 and Hutus held since
then the power, due to being wide majority. Only in 1990, the Tutsi
Rwandese Patriotic Front, supported by Uganda and Burundi, started to
reverse the situation, leading to the civil war and the subsequent
genocide.
Burundi instead, despite the downfall of monarchy in 1966, was kept
under Tutsi hegemony by a series of dictators. In 1993, Burundi finally
held elections, won by the Hutu Front for Democracy in Burundi. Yet
soon a civil war started as well.
See:
http://en.wikipedia.org/wiki/Rwanda
http://en.wikipedia.org/wiki/Burundi
http://en.wikipedia.org/wiki/Hutu
http://en.wikipedia.org/wiki/Tutsi