Spartans are the ultimate super-soldiers of humanity. It is in no small part to them that humanity managed to survive the Human-Covenant War. It is their dark past, however, that makes them such interesting characters.
Spartan-II candidates were all abducted from their families around the age of six in 2517. They then spent the next eight years in training on the planet Reach, culminating in putting all candidates through a physical and mental augmentation process that killed nearly half of all of the fourteen year old soldiers. Though there were only a few dozen Spartan-II soldiers left after the augmentations, they still proved instrumental in humanity's eventual victory against the Covenant.
The Spartan-III program began in 2531, and forwent kidnapping children from their homes to recruiting child orphans from the war. While the candidates selected were not of the same genetic caliber as the Spartan-IIs, they still received the same augmentations, though as a much better success rate. In all, nearly 1,000 Spartan-IIIs were created, though nearly all died in suicide missions in 2537 and 2545.
The Spartan-IV program began after the end of the Human-Covenant War, and replaced using child candidates with adult members of the military. These candidates, while not receiving the same level of training as the Spartan-II or Spartan-IIIs, were nonetheless the pinnacle soldiers in the UNSC, and were used extensively to police the galaxy following the dissolution of the Covenant into several violent factions.