The 1994 Assault Weapons Ban banned a list of specific rifles from importation/sale, including such guns as the Mac10 and the Striker shotgun.
It also banned guns based on certain 'features'. Per the law, a gun may not have more than TWO of the following features and still be legal:
-bayonet lug
-pistol grip
-colapsible stock
-flash hider
-removable magazine
-grenade launcher
With the exception of the last one (since grenades are illegal for civilians to possess anyway), most military style sport rifles manufactured and sold at the time had more than two of these features and were to be banned (note that none of these features (with the exception of the grenade launcher
) change the basic functioning and lethality of the rifle.)
The immediate result was that after the ban, manufacturers simply removed enough features so that now they only have two of the 5 they used to come with (almost always the pistol grip and removable magazine). Pre-ban rifles were grandfathered in.
Manufactures of pre-ban AR15's included Armalite, Colt, and Bushmaster, to name a few.
The AWB also banned the importation and sale of magazines that held more than 10 rounds except to law enforcement and the military. Since pre-ban mags were going to be grandfathered, manufacturers just starting running 24 hours a day, seven days a week producing 10+ magazines before the ban took effect.
In practical terms, there is no difference between a pre-ban and post-ban except that right now, a pre-ban will still cost more than a post ban. That will be the case until the 1994 AWB sunsets in 2004 if it does, (and something else is not passed in its place.)
If it sunsets, manufacturers will start putting out guns with pre-ban features in anticipation of the rush. Also as a result, those that bought pre-ban rifles as investments will lose money.
So right now, many pre-ban owners (and those with 10+ mags) are trying to unload their equipment.
Hope that helped.