Order

An order is an instruction you send to your broker to buy or sell something — shares, contracts, coins — on your behalf. It is the basic unit of action in trading: every position you open or close starts as an order.

An order needs at least three things to make sense: which instrument (say, a particular stock), how much (the quantity, or "size"), and the direction (buy or sell). Beyond that, you choose the type of order, which controls the price behavior. A market order says "execute right now at whatever the best available price is." A limit order says "only execute at this price or better." There are other variants — stop orders, stop-limit orders, and so on — but all of them are really just different sets of rules layered on top of that same buy/sell instruction.

Once you send an order, it isn't a trade yet. It sits with the broker or exchange until it can be matched against someone willing to take the other side. If that happens, the order is "filled" (fully or partially) and becomes an actual executed trade. Until then, it can also be modified or cancelled, assuming it hasn't already started filling.

The nuance that trips people up: an order is a request, not a guarantee. A market order will almost always fill, but not necessarily at the price you saw a second earlier — that price can move between when you click and when the order reaches the market. A limit order guarantees the price (or better) but not that it fills at all; if the market never reaches your price, the order just sits open, or expires, or gets cancelled.