Genocide is the systematic extermination of a group of people who share race, ethnicity, beliefs or other traits. It tends to be organized by either the government (in the case of the Holocaust) or by paramilitary groups (in the case of Bosnia). Often, the conflict is civil in nature and so other nations or groups are reluctant to get involved.
To learn more about genocide throughout history, check out Facing History and Ourselves. It is a great organization that offers quality materials to students and educators about the Holocaust, the Civil Rights Movement and a variety of other genocides.
Genocide is the deliberate and planned killing of an entire group of people, such as a racial, ethnic or religious group. The killing of the Jews by Hitler during the Holocaust was genocide.
It can also be referred as the systematic killing of a large population. Meaning you are following a pattern and a plan for the killing. That's the difference between genocide and other killing which are not organized (massacres, such as war). The people who are being killed don't have to necessarily to a different racial, ethic, or religious group, because the most resent genocides have been committed by political disputes.
Ps: That doesn't mean that Hitler attitude towards Jews in the Holocaust didn't end up in the biggest genocide of the world.
Someone does not have to kill a group of people to committ genocide; genocide includes the stipulation of intent--so killing members of a group or creating conditions upon members of the group with the intent that those deaths or conditions indicate the intent to destroy the particularly defined group.
So, for instance, the death of 300,000 people of a national group is not necessarily genocide--genocide exists only if the killings are intended as an end in itself: the destuction of the group. If the killing are a means to an end, for instance, for the control of land or the gain of political power, then that's not genocide.
If however, a few people are killed or suffer conditions by another that mark the other's intent to kill the group defined by people, then that would be genocide.
Adopted by Resolution 260 (III) A of the U.N. General Assembly on 9 December 1948. Entry into force: 12 January 1951.
Article II: In the present Convention, genocide means any of the following acts committed with intent to destroy, in whole or in part, a national, ethnical, racial or religious group, as such:
(a) Killing members of the group;
(b) Causing serious bodily or mental harm to members of the group;
(c) Deliberately inflicting on the group conditions of life calculated to bring about its physical destruction in whole or in part;
(d) Imposing measures intended to prevent births within the group;
(e) Forcibly transferring children of the group to another group.