SOLDIER. Sephiroth was created from Shinra's desire to harness the power of a half-human, half-Cetra being. Seeing Sephiroth's superior strength and enhanced abilities, Shinra wanted more super soldiers, thus the creation of SOLDIER. Qualifed individuals were subjected to exposure to Mako and injected with the cells of Jenova. To be qualified, one must be amazingly strong mentally and physically. Cloud was an example of what happens to those subjected to this treatment who are not mentally capable; Jenova, controlled by Sephiroth, was able to take control of his mind and body.
RANKS. There are three ranks within SOLDIER:
SOLDIER: Class 1st: This is the highest rank. Though Zack and Angeal wear the dark blue/black (officially, the uniform is listed as black, but in pictures it looks dark blue). However, others like Sephiroth and Genesis wear what they wish.
SOLDIER: Class 2nd: The uniform is a deep wine/dark purple color. Kunsel and Luxiere are both Class 2nd and wear helmets, however, Zack did not when he was Class 2nd.
SOLDIER: Class 3rd: The uniform is blue/navy blue.