One-shot, Drabble, and Short Story Contests

I was looking through contests trying to find something to join in when I noticed tons of:

* Drabble
* One-Shots

I looked everywhere for an explanation on these two.
Basically they are both a one chapter thing and kind of like a short story.
Can someone explain the difference between them? If it were me I would COMPLETELY say that a short story, one-shot, and drabble are all the same. Is that right? Wrong? Maybe both?

I really wanna be able to do this in contests but I can't if I don't understand them.
Anyone willing to help me out in detail without over-complicating this?

ALSO if you know a good contest for beginners on drabbles and one-shots I'd be happy and appreciate it!

-THANKS MIBBIANS!!