The variety of things. Where not everything is the same, but most things are different from each other.
Most often used to describe human societies. A diverse society, workplace, or population consists of a number of different races, religions, classes, opinions, and the like, mixed in together.
One of the keys to being diverse is that there is no seperation or segregation based on those differences. If each group keeps to themselves and does not interact with the others then there is not much diversity.