A guided shell is a piece of artillery ammunition that can alter its course after being fired, in order to hit an intended target more accurately. The most common means of controlling the flight is for the shell to use small vanes or fins that pop out after firing, and provide aerodynamic control.
Guided shells are more than simply ammunition with intelligent fuzing, such as the proximity fuze. They must be able to sense either their target, or a location in space, at which they are aimed, and compare their actual flight path with the optimal one. If the actual path deviates from the currently best one, which certainly can change if the target is moving, corrections are needed.
As mentioned, aerodynamic controls are the most common, but it would be possible to use gas jets or small rockets. To be a guided shell rather than a guided missile, however, rockets cannot contribute to the forward motion of the shell, only to its side-to-side movements and angle of descent.
Guided shells have, in the past, been prohibitively expensive; the United States stopped production of the effective laser-guided M712 Copperhead shell due to its high cost. Advances in electronics and production techniques constantly lower the cost, and guided shells are becoming more common, even in weapons such as mortars.
Laser or GPS guidance are most common.