As for ARs and the like...
A forging involves taking a "plug" or a "slug" of the metal you want to use for the final product and smashing it into shape between two dies (molds). Usually, for the harder materials, the "slug" is heated to red-hot in order for it to flow into the form better. Most of the crankshafts in your cars' engines are made this way, along with some of your more expensive, hi-performace pistons. Sometimes it takes 3 or 4 molds to make the part. Parts are then machined from there. This involves the removal of a lot of 'flashing', with thick parts needing removal in most cases. A "true" forged part is the strongest, as the metal is forced (to a point) to 'flow' into the final shape. The crystalline structure of the metal is kept intact.
Most all of the stuff I have seen has been "investment cast" which is a VERY OLD process. Parts are made by pouring the molten aluminum into sand-cast forms (kinda oily sand), then they are machined from there.
Usually, to make good investment casts, the mold will spin and enact centrifugal force into the process of making the metal have less voids.
SOME (but not many) of the parts out there are from billet, which is almost as good as a true forging. A billet is basically a slab of metal cut from a sheet or extrusion (very high quality). Every single little detail on the billet part is machined into shape...everything.
Honestly. I think forged parts kick-ass over cast parts any day, at least for aluminum stuff.