Kohen – Priest
Kohanim – Priests
Kohen Gadol – High Priest
The first Kohen was Aaron who was from the tribe of Levi, and Hashem changed his genetics to start the line of Kohanim to serve the Holy, and be a channel of Light to the world.
Kohen represents the Right line of the Tree of Life that is Chessed.
Levite represents the Left line, Gevurah.