There are five schools of skills in Phantom Dust.
* Psycho - Represented by an eye in a pyramid.
* Optical - Represented by interweaving curved lines.
* Nature - Represented by a plant.
* Ki - Represented by the yin/yang symbol.
* Faith - Represented by an eye on its own.
All skills belong to a school based on a core concept of how it works. If you play the single player, you'll unlock this somewhere in the second chapter. If you instead go into the multiplayer first, rotate skills every so often and try each school at least once. This achievement will unlock once you use your first skill from whatever fifth school you have left over.