UNSC AI #23 Is definantly right... Seriously, if you guys are gunna come in here all guns blazing trying to prove everyone wrong at least do your research 
Ok, the original Spartans that the Halo Spartans were named after were ancient Greeks. Fought the Athenians etc, were brought up from a VERY young age to be soldiers. One small group of spartans killed thousands of Persians in the brief time that the Greeks were united, and this ultimatley won them the war...
Spartan-I's, the first time (that i am aware of) that these guys were mentioned was in the ilovebee's stories, they were willing participants in the Spartan-I project, not much is really known about these Spartans as they were before the MC's time. One could guess though that they had very minor augmentations and were trained to be elite warriors, best of the best...
Spartan-II's, the Master Chief was the squad leader of these guys, if you read the books you get the full story. Basically, 75 children were kidnapped by the army when they were 6, taken to REACH and trained to be soldiers. At 14 (i think) they were put through an almost experimental augmentation process, resulting in half of the spartans becoming either deformed or just dying from they process. The remaining 30 something Spartan-II's went on as usual. Once the covenant glassed Harvest, the Spartan-II's were taken to a military facilty and given the MJOLNIR mk III armor. It had no shields, however it improved their reflexes and strength and speed almost as drastically as the augmentation had. The Spartan-II's were the only humans capable of controlling the armor due to this (as it describes an ODST attempting to use it being torn apart). The Mark III armor had no shield. When the Spartan-II's were sent onto their mission to capture a prophet, Dr Halsey gave them the Mark IV armor which, using captured covenant technology, had a shield system, which makes the Halo's we all know and love. I won't tell you anymore in case anyone trying to prove people here wrong hasn't read the book yet...
Spartan-III's, well, i can't really say much here because i can't get a hold of Ghosts of Onyx yet, so i'll let someone else *cough UNSC AI #23 cough*