Better Than 223

The actual barrel. Not the length of the rifle. This is what the ATF measures

Barrel material. Could be stainless steel. Could have carbon fiber wrapped around it (for $$$). This is standard rifle barrel material for a 5.56

Nitride is a coating that resists scratching, rust, and is typically black in color

This is in contrast to the "Pencil" barrel that was much more easily bent. This type of barrel was in the M16A2 rifle. There other types of barrels. There is even a DMR - Designated Marksman Rife barrel.

Chamber is where the bullet sits prior to firing. This is not in the magazine. The chamber of the rifle is designed to handle a maximum amount of pressure. 5.56 has 3000+ psi pressure than 223. The 5.56 has a longer throat on the case to allow for a little more powder. More powder = more pressure. 

This describes the amount of rotation per inch that will happen when a bullet travels down a barrel. The smaller the second number, the more times the bullet will rotate before exiting the barrel. Rotation is accomplished by a spiral set of grooves in the barrel. These are the low points. The high points, the lands, are what actually touch the bullet.  So for every 7 inches of length, the bullet fully rotates 1 time as it travels down the barrel. For 1:8, the bullet must travel 8 inches to rotate one full time. So, in a 16" barrel with 1:7 twist rate, the bullet will have rotated 2.285 times. In a 16" barrel with a 1:8 twist rated, the bullet will rotate exactly 2 times. Heavier bullets need greater rotation in order to stabilize properly. 

This is the most popular currently. This extension lowers the feed ramp for the bullet to travel on its way to the barrel. 

For your AR15 pattern rifle, there is a metal tube that is often 0.750" in diameter. When you fire a round, the gas travels down the barrel.  Some of that gas is routed into a 'gas block.' The gas block, as you guessed, blocks the gas from leaving the muzzle. Instead, the gas is forced into the metal tube and travels back towards the bolt carrier group. This fast moving gas is what helps force the bolt carrier group backwards to eject a spent case. Thing of it as a small gas return system that enables your rifle to operate properly. Mid length is where the gas block sits on your barrel. Rifle length is longer than mid-length. Mid-Length is longer than carbine. Carbine is longer than pistol length. If you have a longer barrel than 16", you will probably need a 'rifle length' gas system. The longer tube helps time the bolt carrier's going backwards to eject a round so that the just fired bullet exits before another bullet is loaded and fired. 

Gas blocks can be threaded, standard, and low profile. The low profile sits lower so that your hand guard over the barrel can be smaller in diameter. 

So your upper portion of your rifle starts out as a big piece of aluminum. A machine either pushes it into a die (forged) or a cnc bit is used to cut (machine) the piece of aluminum to shape. Typically, forged are stronger. But many prefer the aesthetic of a billet upper.