How do you choose between limit and market orders?
A large market order will sweep through multiple levels of resting orders, so the average execution price may differ from the price you saw when placing the order. This is slippage.
The thinner the order book and the larger the order, the more noticeable slippage tends to be. For major trading pairs with good liquidity, slippage is usually minimal.
A quick look: $BTC is quoted at 74833.54 (24h -0.83%).