Stop market orders are placed at a specific price, and if the market price reaches the order price, the order should be triggered and executed as a market order.

Market orders are filled at the best available price, which may not necessarily be the price where your Stop Market order is placed in a fast moving market.  This is where slippage can play a larger role in your trading.  

A stop limit order is a stop order which becomes a “limit order” if and when the market reaches a designated price.

Because Stop Limit orders become a Limit order if and when the market reaches the designated price, they may help reduce slippage. Limit orders are filled at the designated price or better.  However, if the designated price is  traded through quickly, your resting Stop Limit order may be converted to a Buy or Sell limit order that is not filled due to the order being too far back in the order queue.