Now to give you a little more info..
heres some ar15/m16 schooling...
BCG= bolt carrier group, in gun terms a group is all the parts that make up something you would call the bolt, when in actuality the bolt is just 1 of many parts that make up the BCG, also trigger group for example the trigger is just the part your finger touches, you still have hammer, diconnector, hammer spring, trigger spring, trigger and hammer pins, disconnector spring ect...
Next for free floating and rail, free floating is when nothing touches the barrel for almost its entire length, it improves accuracy.. The only things that would touch your barrel beyond the barrel nut is your gas block which is only connected to a tiny gas line that runs back to your receivers gas port...
Full rail and flat top and carry handle are pretty self explainitory, carry handle is the old looking style m16ish upper, flat top is the rail look, full rail is when the uppers rail connects non stop to your front handguards rail...
Monolithic is when your front handguard and upper receiver are all one piece, machined out of one piece not separate parts...
push forward assist is that little button that assists the round into a hot chamber when they get sticky, you can get uppers with or without...
Low pro gas black is that little block on the front of you gun where the old ar15 and m16s have the front sight incorporated into it... low pro allows you to have your rail and handguard go all the way forward over the gas block...
Next is gas or block length , this will be shown as carbine which is the shortest, mid wiich is the middle, and rifle which is the longest, these dont have much effect on you since you are going to stay around 16 to keep the gun a rifle, I would go mid...
I would be careful about what length barrel you get anything under 16" is illegal unless your lower receiver is marked and registered as a pistol, so the shortest you can go is 16 total, you may get away with a 14.5" barrel and a 1.5:" muzzle break or flash supresser { if they are legal in your area} as long as it is welded and pinned and no longer removable...
Gas piston uppers are another entire whole world, how this works is like an AK47 instead of hot gasses going from the barrel to the receiver, and blowing all kind of gas and particles into your receiver through your bolt group, making the gun super hot and dirty, a gas piston upper uses a small piston assembly to cycle the action, and this leads to a much cooler and cleaner gun, I like them but they are costly, my favorite is the adams arms assembly...
here is a very affordable gas piston upper, I own this upper, its awesome and I buy from Westin all the time, Cody is a really good guy...
Adams Arms 5.56 16 Mid Piston w/ Magpul MOE Handguard
if you dont like the gas piston idea dd is one of the best bang for buck uppers on the market, I own this one, and what a piece of work it is...
http://www.westernsport.com/product-p/aa-ua-16-m-b-556-dd-10002.htm