A trader dealing on an existing price in the market.
The trader has a sense of urgency.
The trader will enter a market order that will buy at the current ask price in the market. Or sell at the current bid price in the market.
When trading in the Forex market through a retail broker this is how all trading happens. Since the broker offer a bid/offer price and a spread through their trading system.
In Forex it is possible to enter limit orders also, but it will not be possible to get a smaller spread because of this. Since the order will only be filled when the bid/offer price set by the broker moves so that the limit order can be filled.
In some markets such as the stock market where there is an order book it is possible to enter limit orders. These limit orders can be inside the current spread and thus narrow the spread.
Some markets offers rebates for limit orders and fees for orders that immediately take the best bid or offer. This is to promote and reward a sort of market making order behavior. Rebates are offered mostly on electronic stock exchanges.