What Is a "One Shot"?

A One Shot is something that stops after the sample finishes, like an orchestra hit.
 
A Loop is something that repeats, like a drum clip.
 
Technically, inside the sample file are tags which indicate where the loop is to occur. In the case of a drum loop, most likely these are on the extreme start and end. But a violin may have loop points in the middle, so you get an attack, a loop that repeats as long as you keep the key pressed, and a release part that plays out when you let the key go.
 
You can even have start points some way into the sample. If you play soft, it will start from there. If you play hard, the player will start the sample earlier and include more of the attack part.
 
But in the case of the samples in the Media folder, I get the impression the categories are more an indication of intended use. If you want to loop an orchestra hit, nothing is stopping you.