Outgroup bias, like ingroup bias, is a simple concept, but one that has very powerful affects on people, societies, and life in general. Ougroup bias is simply the tendency to have negative views about people that are not part of one's own group. The groups can be any groups you associate with at a particular time.
So, for example, when you play on an intramural softball team that meets once a week, you are part of that softball team's ingroup; people who are on the other teams are part of the outgroup. Or, it can be something on a much more grand scale like, the situation between religious groups in Ireland. They have been killing each other for years, because they each perceive their own group as being the "right" and "good" group, while the other group (the outgroup) is "bad" and "evil". See also: Ingroup