Spartans are genetically-enhanced super-soldiers bred for the combat of war in the Halo universe. Spartans are created through the SPARTAN Program. There has been three different SPARTAN Programs, SPARTAN-I, SPARTAN-II, and SPARTAN-III.
Not much is known about the SPARTAN-I program. It was created to test the effects of genetic enhancements given to volunteers that were part of the armed forces. There are not any active members of the SPARTAN-I program that we know of. All of the ones that we know went through it have either been killed-in-action, or are currently retired. Some people speculate that Sgt. Johnson may have been a SPARTAN-I.
The SPARTAN-II was the follow up to the original program. The SPARTAN-II program incorporated technology along with genetic enhancements to create even better soldiers. The most famous SPARTAN-II is the John-117, the Master Chief. The majority of the SPARTAN-IIs were killed during the battle of Reach.
The SPARTAN-III program is the latest Spartan training program. SPARTAN-IIIs are not as powerful as SPARTAN-IIs in terms of strength, speed, etc. Rather, this program focuses on quantity, survival rate, and economic efficiency.