Mommy, what's a genocide?

My five year old daughter picks up on everything, and asks very intelligent questions. We all want to achieve that delicate balance between telling our children the truth while shielding them from its ugliness. I volunteer my time for the ELCA World Hunger Appeal. I coordinate the program for a portion of the NEPA Synod. I watch films and read books about the problems facing women and children in developing countries. One of her favorite books is Listen to the Wind, which is the children's book about Greg Mortensen (Three Cups of Tea). Recently, I allowed her to watch the competition portion of War Dance, which I highly recommend to anyone who questions whether there is hope in war torn areas of Africa.
Last night, she asked me "What's a genocide". It strikes me that many Americans are not even aware that there continue to be genocides throughout Africa. We have legislated and assimilated in this country. The thought of one tribe maiming and killing every single member of another tribe for no reason other than their tribal identity seems barbaric and inconceivable. Regrettably, with two endless wars of attrition, a decimated economy, and the constant bickering and back biting over what should be a basic American privilege (health care), children high on drugs hacking the shit out of other children on drugs continents away is not a media priority.
Not that it's any surprise. The Holocaust went on for YEARS in supposedly civilized Europe, and America knew and cared very little about it until the liberation of the camps at the end of the war....after millions were exterminated. So it's a given, I suppose, that very few people even know about the genocide(s) occurring in Uganda and Sudan. And that no one did anything in Rwanda until it was over.
I told her that genocide was when one group of people killed or hurt another group of people because they were different from the killers' group. She thought for a second, and asked me, "Why would they do something like that?" Bunny, I wish there were an answer.....