Masters is my catch all phrase for all the various Master technologies, it doesn't exist in actual TF lingo.
Micromaster they are Transformers designed to be the size of humans so that they need less Energon to function. Some stories have it being developed on Cybertron jointly by Autobots & Decepticons before the Transformers wake up on Earth some have it developed based on post Earth timeline (not sure if that was a joint effort). Transformers that are Micromasters were not always created that way, and those that were downsized were not always willing. IMO they are basically the G1 version of
Mini-cons minus the ability to
powerlinx with a full sized Transformer.
Headmaster in the show & comics is typically are humans or
Nebulans who enter the Transformers head and can pilot the Transformer to some degree. There are instances where it is a human sized Transformer (not a Micromaster from what I can tell) that becomes the head for a lifeless body called a Transtector. A Transformer bonded with a Headmaster can transform from alt mode to robot mode without the Headmaster partner being present but it has no head and generally can't do much at all.
Targetmaster in the show & comics is typically are humans or
Nebulans who wears a suit of armor that becomes a blaster for a Transformer. There are instances where it is a human sized Transformer (not a Micromaster from what I can tell) that becomes the weapon instead. A Transformer bonded with a Targetmaster can transform from alt mode to robot mode without the Targetmaster partner being present but they are missing some or all of their firepower depending on what other weapons if any the Transformer has.
Powermaster in the show & comics is typically are humans or
Nebulans who wears a suit of armor that becomes an external power supply for a Transformer. There are instances where it is a human sized Transformer (not a Micromaster from what I can tell) that becomes the power supply instead. A Transformer bonded with a Powermaster can't transform from alt mode to robot mode without the Powermaster partner being present.
There are Mini-cons that act as Headmasters as well as Targetmasters but they are never referred to as Headmasters or Targetmasters. Plus the Mini-cons are not binary bonded to any particular Transformer. Can't recall any Mini-cons acting as a Powermaster.