It's the same as the buy and sell order but differ on the price when you create a buy order, it deducts a little amount; but a little goes a long way right? Whereas when you use the stop-limit, whatever amount in btc you buy; the same amount shows on your buy order. But a lot of people don't actually use much of this feature plus poloniex has been lagging lately.
No, fees apply to all types of orders whether its a market order or any form of limit limit orders. But the fees are lower for limit orders in Poloniex. Im not sure about the other exchanges.
