A limit order instructs a broker to buy or sell only at a specified price or better — a buy limit executes at that price or lower, a sell limit at that price or higher. Unlike a market order, it guarantees the price (or a better one) but not that the order will fill at all: if the stock never reaches the limit price, the order simply sits unfilled until it's cancelled or expires.
Limit orders are the standard tool for controlling exactly what you pay or receive, and for setting a price target in advance without watching the market continuously. The trade-off is the real possibility of a missed opportunity if the price moves away without ever touching the limit.
A limit order and a price alert solve related but different problems: an alert notifies you when a level is hit so you can decide what to do next; a limit order acts automatically the moment that level is reached.