Each has its own unique “users”, the main overlap, is that the flight controls are powered by each system, and in the event that only one hydraulic system remains, it can maintain full control of the aircraft.
Green is powered by a left engine driven pump.
Yellow is powered by a right engine driven pump, with an electric pump backup.
Blue is powered by an electric pump, with a RAT (Ram Air Turbine) backup.
In addition, there is a PTU (power transfer unit) in which Green or Yellow can power each other, if there is a difference between the two systems by more than 500psi.