Awhile back I bought a new Springfield Mil-Spec 1911A-1. I bought 100 rounds of ammo with the gun. I took what the guy gave me and didn't think anything of it. Today after work I stopped a couple places to price more ammo. I walk into the first store with ".45ACP" in my head. I start looking at the ammo and most of the boxes say ".45AUTO". Same at the next 2 stores. I came home and dug out my old boxes and it was ".45AUTO" he sold me. I'm confused. I thought I had to buy ".45ACP" and anything else was different. Like ".38" and ".38SPECIAL". Am I correct or is all .45 ammo the same?

Is this what Walmart sells for $20?

.45 auto and .45 ACP are the same.  ACP stands for Automatic Colt Pistol.

Glock has been involved in a new round, called .45 GAC It should NOT be loaded into guns chambered for .45 ACP.

In .38 there is .38 special and .38 super.  different guns.  

There is .357 magnum and .357 sig (also called .357 auto) different guns

9mm is called 9x19, 9mm luger and 9mm parabellum.  same thing

.380 ACP also is called 9mm short or 9x17 or 9mm kurz    same thing

There is also a round called "45 Auto Rim" which you won't run into to often but it is not the same as 45 ACP, which is what your gun uses.

Yep, good responses.  It's also good that you want to be SURE.  When it comes to firearms, you want to be as SURE as possible.  We're all safer that way.

But yeah, 45 Auto and 45ACP are the same.  I don't know of any other names for the cartridge, so anything labelled as either one of those two should work fine in your pistol.

If you ever get any other pistols, definitely be sure you use the correct ammo.  Definitely ask if you have to.  

Hey now, don't forget the .45 long colt, or just .45Colt (a rimmed revolver round, and also not what goes in your gun).  Just stick with the .45 Auto/ACP and you'll be fine.  All the rest are relatively rare.

Luckily, the other mentioned .45 cartridges wouldn't chamber anyway, as the .45WinMag and LC are too long and the auto rim has a rimmed case head, preventing full chambering.
.45 +P is acceptable also but a hotter loading.
Good gun, keep it well fed.

Just to make things super muddy
.45ACP= .45 auto colt pistol= .45 auto
but don't buy .45 colt ammo, that is for a revolver

Also don't use 45 Super.
Exact same size as a 45 ACP, but loaded much hotter and the cartridge walls are thicker than a standard 45 ACP.
Other than the headstamp saying "45 Super", you'd never know the difference until the slide was sticking out of your forehead.