What is the difference between the standard AR-15/M16A2 rifle receiver extension and the CAR-15/M4 Carbine collapsible receiver extension?

I have not seen this difference mentioned in buttstock catalogs.

Standard receiver extension is a longer (~10") tube that is threaded at one end and has a lip on that end.  Other end has a threaded hole (where the full size stock is screwd in).  It is installed by using a wrench on the far end.

CAR/M4 receiver extension is a shorter (~8") tube that has a rail running along the bottom with holes in it (detents for the sliding stock).  The far end of the tube has a small hole.  This receiver extension needs to addional parts to function; a castle nut and an end-plate.  The endplate is used to keep the receiver extension in alignment (rail down) and to keep the rear take down pin detent/spring in place (this task is done by the fixed stock on the rifle).  The castle nut is used to secure the assembly in place.  You need a special wrench to tighten the castle nut.

I suggest downloading the -23&P (Unit Maintenance Manual) from AR15.com (books section) and seeing how they install that will show you differences graphically.