Hydraulic disc brakes
Hydraulic disc brakes do not use brake cables like a standard bicycle brake, but rather fluid filled chambers and tubing. These brakes are like the brakes on your car. The idea is that there is less energy loss because there are no cables or housing to 'flex' when the brake lever is pulled.

The drawback to this much technology being used on a bicycle is the fact that the brakes become a part of your bicycle that is no longer easy to service. If you develop a fluid leak in a line or a fluid chamber out on a ride, your brakes no longer work and there's no way of servicing them 'in the field'. Servicing the brakes becomes an expensive procedure that can only be performed by experienced mechanics.

Wrap up
If you think of the bicycle as a simple machine that should be relatively inexpensive and easy to service, then hydraulic disc brakes are not the choice for you.