Market order: buy or sell a given instrument at the market price.
The price for this type of orders is defined as the best price available for the market at the moment the order is being placed.
Since the price changes in real-time, the total and fee are provided as estimates rather than exact values.
Below is example with screenshot of order book. The best available price is displayed in the top line, highlighted in red: 14913.22 USDT per BTC. If you set up a buy market order at this moment, you'll be paying this price for your trade.
But there's only 0.10850 BTC available at that price. If you buy bigger amount, you will be paying the second best price of 14913.26 for that "extra".
So that, if you were to buy, 0.40 BTC, for example, you would in fact purchase:
0.10850 BTC for a price of 14913.22 (first line in the order book)
0.10000 BTC for a price of 14913.26 (second line in the order book)
0.19150 BTC for a price of 14914.07 (third line in the order book)
0.40 BTC total
Stop order: execute a trade at a specified price (stop price).
A trader may specify the desirable stop price for this order. When the stop price is reached, a stop order becomes a market order.
A stop order doesn't require reserved funds and will not appear in the order book before it's activated.