From Wikpedia...
http://en.wikipedia.org/wiki/Assault_rifle
The assault rifle is a term describing a type of automatic rifle generally defined as a selective fire rifle or carbine, chambering intermediate-powered ammunition. They are categorized between the larger and heavier light machine gun, which is intended more for sustained automatic fire in a support role, and the smaller submachine gun, which fires a handgun cartridge rather than a rifle cartridge. Assault rifles are the standard small arms in most modern armies, having largely replaced or supplemented larger, more powerful rifles ('battle rifles') such as the WWII-era M1 Garand and Tokarev SVT. Examples of assault rifles include the AR-15 and its military incarnation, the M16 rifle, the ubiquitous AK-47 genus, and the bullpup FAMAS. More exotic examples include the exquisite LR 300 By Z-M Weapons and the HK G36, which makes extensive use of modern polymers and space-age engineering.
The name is a translation of the German word Sturmgewehr coined by Adolf Hitler to describe the Sturmgewehr 44, the firearm generally considered the first true assault rifle and effective progenitor of the concept. It gradually became the common (if not technically accurate) term for the sub-family of similar firearms, and also has been used retroactively with earlier weapons possessing such traits. While it is not a completely accepted or technically correct label, it is widely used to differentiate current small-caliber weapons such as the AR-15 and AK-74 from earlier semi-automatic or select-fire rifles that fired larger cartridges, such as the M1 Garand, M14, HK G3, CETME, and FN FAL, which are sometimes referred to as 'battle rifles'. Technically, all are 'rifles' of different caliber, with some being semi-automatic or having select-fire capability, thus being either 'semi-automatic rifles' or 'automatic rifles'. Since the smaller-caliber weapons tend to be distinct in construction and modern use from their counterparts, the term 'assault rifle' has proven useful and popular for referring to them specifically.
...some history of the various assault rifles followed by the US use.
"Assault weapons" vs. Automatic weapons
Primarily in the United States, the term assault weapon is an arbitrary (and politicized) phrase generally used to describe a variety of semi-automatic firearms that have certain, mostly cosmetic features associated with military/police use. The 1994 Federal Assault Weapons Ban in specific, which was largely responsible for forming the American public perception of what assault rifles were, dictated that such features as a folding stock, flash suppressor, bayonet lug, pistol grip, night sights, muzzle grenade launcher, or the ability to accept a detachable magazine of a capacity larger than ten rounds constituted a firearm being an assault weapon. While that law, the sometimes-called "Clinton gun ban" or "1994 crime bill" expired in 2004, some states such as California and New York adopted similar bans, and frequent unclear representation, misrepresentation, or misunderstanding of what exactly was banned and what an 'assault rifle' is have continued to influence the American public perception of assault weapons. A common misconception persists that the assault weapons ban restricted weapons capable of fully-automatic fire, such as assault rifles and machine guns. Fully-automatic weapons were unaffected by the ban; they have been heavily regulated since the National Firearms Act of 1934. Subsequent laws such as the Gun Control Act of 1968 and the Firearm Owners Protection Act of 1986 also affect the importation and civilian ownership of fully-automatic firearms.
The term 'assault weapon' is frequently used erroneously to describe machine guns. Many states and localities still use the term assault weapon with a handful of variations following the California model loosely. The term "spray" of bullets is a good example of that mischaracterization. Semi-automatic, so called "assualt weapons" fire no faster than their semi-automatic shotgun and deer rifle cousins. See separate article on assault weapons for further information.