Glad you asked.
The .45 ACP (Automatic Colt Pistol) was developed for the US Army for Browning’s renowned 1911/1911A1 autoloader almost 100 years ago. Fundamentally (there are several exceptions to this generalization, but I’m trying to provide the “basics” is this reply), it is used in semiautomatic pistols. Normally, the .45 ACP has a mass of 180 to 230 grains and a muzzle velocity approximating 850 FPS. It epitomizes the “big, slow” school of handgun shooting; many individuals -- I am one -- feel it is the “standard of comparison” for all semiautomatic sidearms.
The .45 Colt (sometime erroneously called the .45 Long Colt) originates from the mid/late-1800s. It is a revolver round and the cartridge is considerably larger than the .45 ACP’s. Initially, it was the “peacemaker” of the Western frontier and was made famous is for its potency in single action revolvers (such as Colt’s Single Action Army model). Despite its age, it remains a very useful and viable revolver round, especially due to its flexibility. More specifically, at low-pressure loadings, it has ballistics similar to the .45 ACP P+ (for example, a 200 grain projectile with a muzzle velocity of 1100+ FPS). However, at its higher-pressure loadings (probably not suitable for many revolvers other than modern Rugers) the .45 Colt’s ballistics approximate the .44 magnum’s -- it is a real “stopper”.
Many people call the far-more-common semiautomatic round the “.45 Colt”, but as you can see, that is wrong and confusing, since they are two distinctly different cartridges.
You may want to check the TFL archives (under “Search”) for much greater and more detailed information. You may also want to review reloading manuals for sizes and the reloading/ballistic specifications applicable to these two rounds.