RC Servos can be rated two types: digital and standard. Both digital and standard servos can be used with a normal receiver, the real difference is performance.
All servos use a series of short pulses as signals that determine what angular position they should maintain. The series of signals is usually very fast, somewhere around 50 pulses per second at maximum. On a standard RC helicopter servo, the rate is so fast that very small movements of the control sticks may not have an affect. This means that there is a “deadband” on the control sticks, in which no servo movement takes place. Although it’s not a problem on trainers and most sport class models, the deadband becomes a significant issue with 3D aircraft. Even a small delay with a 3D aircraft could cause a severe crash.
Digital servos remove the deadband by speeding up the rate at which it receives pulses. Usually, this is increased from around 50 to 300 pulses per second. This increase in resolution allows the servo to operate much more precisely.