Market order: buy or sell a given instrument at the market price.
The price for these types of orders is defined as the best price available on the market at the point of time the order is being placed.
Since the price changes constantly, the total price and fees are provided as estimates rather than exact values.
Please note, when trading less liquid assets, market orders might not be the ideal choice. A better option are limit orders that let you specify a price at which you're willing to buy. For more information on this order type, read here.
Stop order: execute a trade at a specified price (the stop price).
Traders may specify the desirable stop price for an order. When the stop price is reached, a stop order becomes a market order.
Stop orders don't require reserved funds and won't appear in the order book before being activated.