Below is a video that explains the parts of the M16 along with all positive features of the rifle.
The production of the M16 started after the US infantry board presented characteristics of a small caliber rifle to USCONARC (United States Continental Army Command), but before USCONARC approved the idea, General Willard Wyman “acted to expedite the development of a lightweight rifle.” Of whom he told was Eugene Stoner who developed the AR15 based on Wyman’s specs of were “a maximum loaded weight of six pounds; a capability of firing semiautomatic or full automatic; a killing power equal to or better than that of the Ml (Garand) rifle up to 500 yards; and a capability of penetrating a steel helmet or standard body armor at 500 yards”. From that rough description Stoner modified his older model the AR10 into the AR15, which was later called the M16 after the US Military started using it.
The AR15 was changed from the AR10 in many ways to fit the description Wyman gave Stoner. It would use a 5.56mm caliber cartridge instead of the 7.62mm cartridge that was being used in the majority of assault rifles at that time. Along with the smaller cartridge the AR15 was also a lighter weight and was about 6 inches shorter than the M14, the current rifle used by the US military. Altogether the AR15 was a more superior rifle than the M14 because of the lighter weight and the decreased size, but more importantly because it used a 5.56mm cartridge, soldiers would be able to carry more rounds with them, therefore giving them an advantage over the enemy. 
Once the rifle was finished and it was tested by the U.S. Army Infantry Board in 1958. The Board said it should be put down as a possible replacement to the M14 rifle.
In 1959 the rights of the AR-15 Rifle were sold to Colt’s Manufacturing Company, who supplied the weapons to the military throughout the Vietnam War. Like all inventions, the M16 Rifle needed improvements made to it in order to improve minor details and malfunctions. Colt made those small changed based on the requests of the U.S. Military. These modifications include different materials used on the interior and exterior of the rifle, changes in the buffering system and changes to the magazine spring along with many others.
 J.D, Randall. “Vietnam War Photos – SEALs.” The Vietnam War. http://www.vietnamgear.com/gallery.aspx?GalleryID=6. (Accessed March 26, 2013.)
 Richard R. Hallock, Colonel U.S. Army. M16 Case Study, Prepared for the Chairman of the President’s Blue Ribbon Defense Panel, March 16, 1970. 3.
 “M-16 Operation And Cycle Of Functioning (1966) – YouTube.” YouTube. http://www.youtube.com/watch?v=bJODc5t7SGY (accessed March 26, 2013).
 Department Of The Army, Report Of The M16 Rifle Review Panel, Office Chief Of Staff, Office Director Of Weapon-Systems Analysis, Ada953110, Washington DC,: Department Of The Army, June 1, 1968, 216, C-4.
 Hallock, M16 Case Study, 20.
 Department Of The Army. Report Of The M16 Rifle Review Panel. C-2.
 Ibid., D-11.