Analog-to-Analog Conversion is the conversion of a continuous electrical signal from one form to another.
An example would be converting between the 60 Hz current of the US into the 50 Hz current of Europe.
Analog to digital
Used in power supplies
involves a transformer to change the voltage and diodes to recitfy the voltage.
Analog to Digital Conversion is the process of making a computerized representation of some physical attribute in the real world. Turning a voltage into binary numbers is the handiest way to do this, the voltage could be manipulated by a microphone transducer, a potentiometer wiper, or the conductivity of prevarication.
Digital to analog
digital to digital