A tractor beam is commonly used to capture loose cargo pods and miscellaneous debris - typically being employed after a battle. A favorite with pirates, it is also used by merchant fleets to transfer cargo between ships and on mining ships extracting raw materials from asteroid fields.
The beam utilizes gravitic technology to attach an ionized energy beam to a target. Once activated, the beam will increase gravitic power, thereby drawing the targeted object closer to the tractoring ship. Once close enough, and the object is sufficiently small, it will be stored away in the ship's cargo bay. Where your ship is not equipped with a tractor beam, but is capable of being equipped with one, find a shipyard and acquire one.
If your ship isn't normally capable of equipping a tractor beam, purchase "tractor capability" and have it installed to a specific weapon mountpoint. This will allow you to then install a tractor beam to that mount.
Using a Tractor Beam
A tractor beam is installed to a gun mount in the same manner as any other weapon. To use a tractor beam, cycle through your guns using to select the tractor beam alone. Target and lock onto your object of interest, use to match speed, to after burn close to the object and fire the tractor beam using to capture the object.
An interesting usage of the tractor beam is where a sufficiently large ship (such as the Ox) employs a tractor beam to capture whole ships (typically fighter craft). Once rendered inoperative, the craft are tractored into the cargo bay and transported for sale at the next point of call.
A derivation of the above, is the use of pre-purchased cargo ships as wingmen. Please refer to Cargo Wingmen for further information.