A unified transition is one in which both feet are used simultaneously at some point in the transition.  This can be done as a either a landing on both feet, a takeoff from both feet, or both.  3 of the 4 unified transitions, the pop, punch, and bound, are extremely common, and often feel quite natural to the beginner and professional alike.  The least utilized, the reverse-pop is itself a powerful transition, when used properly, but sometimes require a bit more finesse and experience to fully utilize its potential.