Jedi/Sith are a class of unit, and are trained at the Jedi/Sith Temple. They are melee foot soldiers that are good against Troopers and some Mechs. Jedi/Sith are the only units in the game that can pick up Holocrons.
The Jedi/Sith Apprentice is a simple foot soldier. It can be upgraded to the Jedi/Sith Knight, which is stronger, and has the ability to convert enemy units. The Jedi/Sith Master is not an upgrade of the Knight, but a separate unit. It's even stronger in combat, and its hit points will regenerate. With the right upgrades, the Master can become stealth, detect stealth units, and convert buildings, Heavy Weapons, Aircraft, and other Jedi/Sith.
All Jedi/Sith are biological.