What is the differences between "Stop" and "Fill or Kill" order?

Stop order is used to trigger a market sell when the market drops to your trigger price or used to trigger a market buy if the market rises to your trigger price.
If shorting, a trader would place a stop buy above the current price. This would mean that if the market goes against their short (up) they can cut their losses by buying to close or reduce their short position.

Fill or Kill - order is a limit order that must be filled immediately in its entirety or it is canceled (killed). The purpose of a fill or kill order is to ensure that a position is entered instantly and at a specific price.