G&P outer barrels attach to the standard G&P M16 / M4 series bodies via the usual barrel nut and Delta ring (or compatible alternatives). At its most basic, a replacement barrel can be used to vary the length of both the inner barrel and overall weapon size of your platform. Part of the charm of G&P compatible ARs is that the barrel lengths can be found from 2 to 20 inches in increments of 2 inches along almost that whole scale.
If you have an M16 and think its too long you can trim it to an M4A! length 16 inch barrel. If you want to upgrade to a longer range more accurate weapon then you can step it up to a 20 inch barrel on a marksmans platform. You can eve snub M4s down to 14, 12 or even 10.5 inch barrels converting them into more agile SBRs (short barreled rifles) or even short then that if you want to build some sort of AR machine pistol sort of thing.
Besides the basics, barrels also come in a variety of styles and colors. Simple description rarely does the subtlety of aesthetics justice which is exactly what Redwolf makes use of a barrage of high resolution photos to let you see it for yourself.
Barrels can either be Aluminium or steel; steel offering a stronger and more realistic mass of barrel but Aluminium offering still pretty good realism with a lower weight and rust resistance.