A market order says 'fill me now, at whatever the best available price is.' A limit order says 'fill me only at my price or better.' That single difference shapes how much control you have over your entry.
Use a market order when getting in or out quickly matters more than a few cents — for liquid names like AAPL or SPY, the price you see is usually the price you get. Use a limit order when you have a target and are willing to wait, or when trading thinner assets where the spread is wide.
The trade-off is certainty vs. price. Market orders guarantee execution but not price; limit orders guarantee price but not execution. Beginners often over-use market orders — a limit a little below the current price frequently fills anyway and saves you the spread.