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...

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