What is a Market Order? [1]
Market order (instant order) involves buying or selling cryptocurrency immediately at the best available price. On CEX.IO for buying with a market order, you select the amount of FIAT to spend (i.e. "Buy me enough bitcoins for $100 right this moment"). Selling with a market order is the same as Limit Order, except the best available price is used automatically
What is a Limit Order? [2]
A limit order is placed to buy or sell a pre-defined amount of cryptocurrency at a set price. Execution of this order type is not guaranteed until a price requested by the placing user can be matched with an opposite order from another user. Until full execution, the order may be canceled by the user at any time.
You would use Marker Order when you want to sell all your coins immediately for the best price you can get for all of them. For example, you want to sell 10 coins at the market. There are buy orders for 4 coins at $10 and 6 coins at $8. You will sell 4 coins for $10 each and another 6 for $8 each.
And you would use Limit Order when you want to sell 10 coins for at least $10 each and nothing less. If there are only 7 coins for this price at the buy order, you will sell those 7 for $10 and leave a order on the market for the remaining 3 coins at this same price to get executed when possible.
[1]
https://support.cex.io/hc/en-us/articles/226949188-What-is-a-Limit-Order-[2]
https://support.cex.io/hc/en-us/articles/226844807-What-is-a-Market-Order-
