A tractor beam is a device that can apply force remotely to other objects. Tractor beams are usually installed on starships and used for towing other starships, although they can be used as weapons if sufficiently powerful. A tractor beam that pushes instead of pulling is sometimes called a repulsor beam.
Tractor beams in Star Trek typically have short range, often less than a kilometer. Tractor beams interfere with transporter function, blocking or even redirecting transporter beams. Tractor beams can also negatively affect the performance of disruptors.
The range of tractor beams in Star Wars is inconsistent. The range of tractor beams on Imperial Star Destroyers (including the Executor) appears to be very short (a few kilometers), but the range of the tractor beam on the Death Star was apparently very long (hundreds or thousands of kilometers).
Civilizations advanced enough to have artificial gravity technology (including the Minbari and Centauri) can use it to construct tractor beams. Tractor beams have never been seen used at ranges of more than a few hundred meters, nor have they been used as weapons.