A tractor beam is an attenuated linear graviton beam used by starships and space stations to control the movement of external objects. The beam places spatial stresses on the object in specific areas allowing the ship using the tractor beam to hold the object in a fixed location or alter its position or trajectory. While tractor beams are normally used to pull objects towards the beam source, or to tow objects, they can also be set to repel objects.
Tractor beams are generally only used at sub-warp velocities. To safely tow a vessel at warp speed, the target vessel's engines must be deactivated to avoid shearing forces against towing vessel. Using a tractor beam can be done at warp speed only if both vessels' speeds are exactly matched.
A very few stations near high-gravity sources, such as those onboard the worldship are capable of moving much larger masses at Warp Speeds following the formula: (local gravity * tractor beam type) / max jerk = max mass. This is possible by retrofitting CenterPoint Station Technology onto the MicroCochrane emitters and tying in the local inertial dampeners.
Tractor beams have also been put to use as weapons. The Borg frequently use them to strip their enemies' deflector shields to enable use of the cutting beam. (TNG: "Q Who", "The Best of Both Worlds, Part II", DS9: "Emissary") In 2409 the freighter SS Shargrash used her cargo tractor beam to fling chunks of debris at attacking Jem'Hadar starships, while the USS Bajor used her main tractor beams to hold smaller vessels in place, rendering them easy targets for her phaser arrays. (B&S: "No, It's Not, Either", "Asymptotic to Death")