NOS = Acronym for "new old stock." These are parts that originally were available from the manufacturer of an automobile. They have never been used, thus are "new."  

A good example of an NOS part for the 86/87 Grand Nationals is the Rear sway Bar   http://www.highwaystars.net/sway-bar-rear-sway-bar...

OEM  = Acronym for "Original Equipment Manufacturer" designates a replacement part made by the manufacturer of the original part. 

An automobile part may carry the designation OEM if it is made by the same manufacturer that made the original part used when building and selling the vehicle. 

A good example of an OEM part for the 86/87 Buick Grand Nationals are the Tridon Air Intake Clamps http://www.highwaystars.net/clamps-air-intake-comp...  

Unfortunately this term "OEM"  is often mis-used by various sellers in the E-commerce world.  It definitely becomes a frustration for buyers, because of these misleading practices.  BUYER BEWARE

AFTERMARKET = The term aftermarket is often used for non-OEM spare parts.  As a buyer, the only confidence you may have before you purchase an unfamiliar aftermarket product, is that assurance you have acquired from past experience with that product or that particular seller. It is often unknown territory.

In purchasing parts at national, discount auto parts retailers, many parts will have OEM prominently displayed.   but really should be followed by a qualifier such as "meets OEM standards" as long as they REALLY meet OEM standards.  

Such auto parts are not OEM; they are simply claiming to have been manufactured to the same specifications as the OEM parts—specifications that may well be unpublished and unknowable. 

Examples of falsely advertised OEM Buick Grand National products, can also  be found all over ebay.  In particular, one of our customers found a listing advertised as: 84-87 New Buick Regal / Grand National OEM Front & Rear Rubber Bumper Fillers"  and purchased it.   Expecting to receive OEM rubber bumper fillers, to his regret he received aftermarket bumper fillers and they were NOT RUBBER and NOT made by the Original equipment manufacturer.  

BUYER BEWARE

Authorized GM Restoration Parts Licensee = GM approved, & contracted companies such as Highway Stars.  

GM Restoration Parts Program is evidence of GM's commitment to the restoration parts market. "Through the program, restoration enthusiasts have access to quality, original equipment parts and accessories produced to specifications by approved, licensed manufacturers using original blueprints and, when available, original GM-owned tooling.