Subject title says it all.

I was looking at an advertisement and the blurb says the product includes 500 loops and more than 1,000 one shots. I assume the loops and one shots contain audio, not midi, but that is just an assumption based on the way the advertisement is written.

So, what is the difference between an audio loop and an audio one shot?

The end point of an audio loop blends seamlessly with the start part of the same loop, or another loop, so for example if you have a drum audio loop that plays for one bar, put four of them one after each other and you now have a drum track that plays for four bars, etc.

A one shot is just that, one shot of audio. A cymbal crash is a good example of a one shot.

Back when BIAB was still MIDI only, before they added realdrums, the first thing I would do after I had finished building a song in BIAB was throw away the MIDI drums and build a new drum track using loops. I used a program called ACID Music, and made extensive use of drums loops and one shots.

To add (a very small amount) to what Dave said...

A "one shot" is a single hit, note or chord. A single strike of any drum (or cymbal)... a strum of a chord on electric guitar (often a held chord) is also popular. A single note on a guitar (not very useful unless you are using it in a sampler to build an instrument - and even then, not very useful)... although the drum hits can be used in drum programs (like EZDrummer) to build drum kits of your choosing...

Loops are generally phrases - and, for most people, a lot more useful...

Okay, I think I understand. Then by these definitions the audio files in Band-in-a-Box's Shots Holds folder(s) would be One shots as well as the sampled hits at the end of many RealDrums audio files, correct?

Exactly.

yeah, one shots are good for building drums (anything that just needs a single sample to replicate the sound of the full sample).

The other difference (in my head) is that One Shots are meant to play all the way through no matter how many times they are triggered. For example a cymbal; you don't want the first instance to die when a second one is triggered.
So to me they aren't looped, aren't pitch shifted on the keyboard/note number, and finish the 'one shot' no matter how many times triggered.

I mention this because (for example) '80s sampled vocals would sometimes be re-trigggered (and previous one stopped) when they were called. I don't consider these true 'one shots' as they don't play the whole shot when triggered.
Art of Noise is an example of cutting the sample on the word Paranoimia for example

(the above is just my take on it, but aspects to consider)