servo motors

Can someone explain to me the various parts of a servo motor for use
in a CNC machine? What is the resolver, encoder, tachometer, servo
amp, and anything else I missed and its purpose? I know what the
encoder is since it reads upto 2000 lines/revolution. It essentially
reads the position. The tachometer is the speed at which the motor
is running. What is the resolver and servo amp? Is there anything
else I am missing to build a complete closed loop cnc using servos?
Also, I want to use a servo motor as a spindle to both drill and tap
with. What type of spindle servo will allow various rpm drilling
speeds and allow the cnc machine to do tapping?

Thanks,