A market order is an order to quickly buy or sell at the available current price. Market order buys or sells right now (immediately).
When should you use it?
Market orders are handy in situations where getting your order filled is more important than getting a certain price. In other words, market orders should only be used if you are in a rush or the current price actually is your desired price. So, if you need to make a trade right away, that's when market orders come in handy.
When you should not use Market order?
If you are planning to make an impressive deal, check the trading volume and prices in the order book. It is important to understand the size of your order and how it may be executed considering available offers in the order book, as well.
If the volume of your trade is large, you run the risk of closing the trade at a price higher than the market price or higher than the last completed trade on the exchange, since the volume of your order can be executed partially or completely by collecting all available offers from sellers.
The solution to this dilemma is splitting your order into several parts to make sure you are purchasing a certain amount of an asset at the price you need, or using a Limit order.