Essentially when each node has its own intelligence for local processing and can be left doing its own thing, only requiring intervention from a central controller when changes need to be made, or for monitoring purposes. Communication is usually by some sort of serial bus. A car is a good example, having many localised processors monitoring brakes, engine etc, all talking through a common CAN bus. Generally appropriate for more complex systems. As to disadvantages/advantages, it is just a matter of what is an appropriate solution.