Definition
An order type is the specific contractual instruction a trader gives a broker about how to execute a transaction. Each type makes different guarantees and accepts different risks. A market order guarantees execution but not price. A limit order guarantees price but not execution. A stop order guarantees execution if a trigger is hit, again without a price guarantee. A stop-limit order requires both a trigger and a price ceiling or floor, and so guarantees neither outright. Bracket and trailing orders are composites built from these primitives.
The choice of order type is one of the few execution decisions entirely under the trader's control, and it determines how every other variable — slippage, fill probability, time at risk, exposure to gap moves — resolves. Picking the wrong type is one of the most common ways a backtest-clean strategy underperforms in production.
Why it matters
How it works
The two primitives are market and limit. A market order says "trade immediately at whatever price the book offers" and is filled at the inside ask (when buying) or inside bid (when selling), walking deeper into the book if size exceeds the inside quantity. A limit order says "trade only at this price or better" and rests on the book at the limit price, waiting for a counterparty. The first prioritises certainty over price; the second prioritises price over certainty.
Composite types build on these. A stop order waits for the market to trade at or through a trigger price, then submits a market order — useful for exits but vulnerable when the market gaps through the stop overnight. A stop-limit submits a limit order on trigger, protecting against gap fills but accepting that the limit may never be reached. A bracket order combines an entry with a profit-taking limit and a protective stop, three legs submitted together. A trailing stop adjusts its trigger as the position moves favourably, locking in gains. Modern algorithmic order types (VWAP, TWAP, iceberg, peg) are higher-level abstractions that decompose a parent order into child orders of these primitive types according to a schedule.