ORDER TYPES: MARKET, LIMIT & STOP
Plain English
A Market Order says 'buy/sell NOW at whatever the current price is.' A Limit Order says 'only buy/sell at this specific price or better.' A Stop Order says 'trigger a market order when the price reaches this level.' Each has a purpose.
Going deeper
Market Orders execute immediately at the best available price — guaranteed fill but not guaranteed price. Limit Orders specify the maximum price you'll pay (buy) or minimum you'll accept (sell) — guaranteed price but not guaranteed fill. Stop Orders (stop-loss) become market orders when a trigger price is hit, used to limit losses. Stop-Limit Orders become limit orders at the trigger price. Trailing Stops follow the price up and trigger if it drops by a set amount. Good-Til-Canceled (GTC) orders persist until filled or canceled. Day Orders expire at market close.
Examples
Market vs. Limit
Stock is showing $50.00 bid / $50.10 ask. A market buy fills immediately at $50.10. A limit buy at $50.00 sits in the order book until someone is willing to sell at $50.00 — it might fill instantly or might not fill at all.
Stop-Loss Protection
You buy a stock at $100. You place a stop-loss at $90. If the stock drops to $90, your stop triggers and a market order sells your shares. You limit your loss to roughly 10% instead of riding it down further.