Every stock trade requires choosing an order type, and the two most common ones trade certainty of execution against certainty of price.
A market order asks for immediate execution
A market order instructs the broker to trade at the best price currently available, without specifying what that price must be.
It will generally fill quickly in an actively traded stock, because there is standing interest on both sides at prices close to the last trade.
What it does not guarantee is the price, which is whatever the book offers at the moment the order arrives.
A limit order specifies a boundary
A limit order states the maximum price for a purchase or the minimum for a sale, and it will not execute outside that boundary.
If no counterparty is willing to meet the limit, the order rests in the book unfilled, potentially for the rest of the session or longer.
The tradeoff is explicit: price is controlled, execution is not, and the order may expire with nothing having happened.
Spreads and depth determine the cost of immediacy
The gap between the highest bid and the lowest offer is the spread, and a market order crosses it to obtain immediate execution.
In heavily traded stocks the spread is narrow and the cost is small, while in thinly traded ones it can be wide enough to matter substantially.
Depth matters as well: a large market order can exhaust the best price and fill progressively at worse ones, which is why order size interacts with order type.
Openings and news are the dangerous moments
Prices can move sharply between the previous close and the opening, so an order entered overnight may execute at a level far from the last observed price.
The same applies around company announcements, when quotes can move faster than an order travels.
These are precisely the conditions where a limit order's boundary does the most work, and where a market order's indifference to price costs the most.
Stop orders are a separate concept
A stop order is dormant until the stock reaches a trigger price, at which point it becomes an order of another type.
A stop that becomes a market order carries the price uncertainty of a market order, which is why a triggered stop can fill well below the trigger in a fast decline.
Understanding what an order becomes when triggered is the part most often overlooked, and it determines what actually happens in the conditions the stop was placed for.