What is voluntary Balkanization?

Voluntary Balkanization is essentially the development of cliques (on a large scale). Groups of people will stratify (separate) themselves out from one another, and ensure that they spend their time primarily with individuals from their own group.

It comes from the context of the history of the Balkans, an area in the middle-east that experienced considerable conflict when a territory splintered into many smaller states, each of which wanted to be separate from one another.