Market order buys/sells immediately at best available price. Limit order waits for your desired price.
Example
You want to buy BTC, currently at €60,000. A market order fills immediately at the best available price — fast, but you pay the spread and risk slippage. A limit order at €59,500 only fills if price drops there — you get your desired price, but maybe no fill at all. Rule of thumb: limit when price matters more than speed; market when you absolutely must get in or out.