The same thing 9mm bullets are made of, and 7.62x39 bullets, and... well, bullets in general. The case can either be brass or steel. The projectile will primarily be lead. It may have some other type of metal inserted into it, and may or may not be jacketed with copper.
.45 ACP.
.45 ACP
Glock pistols are made in a variety of calibers. Currently available in .380 ACP, 9mm Luger, .357 Sig, .40 S&W, 10mm Auto, .45 ACP and .45 GAP.
Yes
Yes. AR-style rifles can be had in .45 ACP, for example. There was also a .45 ACP variation made of the Uzi Model B (good luck finding one, though). And there are others, as well.
9x19 Parabellum 9x21 IMI .40 S&W .45 ACP
You can fire a .45 ACP cartridge through a .45 ACP pistol - and only a .45 ACP pistol. The various .45 cartridges (.45 ACP, .45 Long Colt, .45 GAP, etc.) are not interchangeable with each other.
The actual power of any given cartridge is based on speed and weight of the bullet. There are multiple different loads for the 9mm Parabellum, and for the .45 ACP. HOWEVER- in general, the 9mm Parabellum has a higher energy than the .45 ACP. Search Wikipedia for 9mm Parabellum, and for .45 ACP. Scroll down for energy figures on the left. It should be noted that almost all .45 loads have heavier bullets, and the .45 ACP is generally considered to have greater "stopping" power than the 9mm.
ACP stands for Automatic Colt Pistol. It is used to differentiate two bullets of the same caliber when one is for a pistol and one is for a revolver. For instance, .45 Colt is for a revolver, and .45 ACP is for a pistol. Other than the .45 inch measurement, the two have nothing in common and are not interchangeable; the .45 ACP cannot be used in a revolver chambered for .45 Colt and vice versa. The cartridge length and capacity are just two of the factors that make each different from the other.
made 1917
about 1961
The .45 GAP is a shorter cartridge than the .45 ACP. Developed for the Glock Automatic Pistol (that's the GAP) it does not interchange with the .45 Colt Automatic Pistol (ACP). Made for a smaller firearm, shorter action, etc.