Agreed with the above in that, in a genocide, the mass murder taking place is systematic rather than random (and usually government sponsored or legitimized), sustained rather than occasional (as in a race riot or pogrom), and targets a specifically identifiable group of people (such as a religion, tribe, or race).
Simply put, homicide is to murder one, and genocide is mass murder, but looking at the recent historical examples - Darfur, Rwanda, East Timor - they all follow a pattern with the elements discussed in the above post and this one.
To me, a genocide is hard to define, but there are a few things that have to happen in order for something to be a genocide.
First, a lot of people have to be killed. You can't have a genocide of just a few people.
Second, the people killed have to be of some particular race or ethnic group that is different than that of the people doing the killing. It doesn't have to be a difference that makes sense, but as long as they see it as a difference and it is racial/ethnic (not political), it can be a genocide.
Third, there has to be an attempt to kill the whole group of people.
Fourth, the killing must be based on the victims' racial/ethnic status.
Genocide refers to mass killing of a large number of people with the intention of extermination of national, religious or other similar groups of people. Genocides are generally committed by governments rather than private individuals.
United Nations Convention on the Prevention and Punishment of Crime of Genocide defines genocide as:
any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such: killing members of the group; causing serious bodily or mental harm to members of the group; deliberately inflicting on the group conditions of life, calculated to bring about its physical destruction in whole or in part; imposing measures intended to prevent births within the group; [and] forcibly transferring children of the group to another group.
Complete text of this convention is available on website referred below.