A tug is a special mostly remote controlled craft, which actuates another craft or modules on different orbits. The main reason of their employment is the option of the leaving the moved module in a passive state. The tugs can neither leave the surface nor land on it, they need a launch vehicle for getting into orbit. Their refueling always happens in orbit by docking to a craft. The tug is typically a reusable craft.
There are two main types of the tugs: orbital and the inter planetary. The orbital tug is kind of "orbital forklift" used for assembling space stations or bigger spaceships, relocating satellites or removing the debris from the orbit. The orbital tugs, able to carry whole crafts for mission to different celestials and return to starting orbit. There are also asteroid capturers. The re-usability doesn't mean strictly single stage: drop tanks also can be attached to the docking ports, and is those drop tanks have control capacity and engines with fuel provision, they can also return for reuse.
The efficiency of the engines of these crafts - beside the proper thrust - has a key role. Therefore the LV-N is widely used at these crafts. The payload has to be attached rigidly, the employment of the multi-ports and the big Clamp-O-Tron Sr. Docking Port is recommended. The stress capability of the port limits the acceleration and the mass of the payload, and so the maximal safe thrust depended on the ratio of the weights. Naturally, the tugs mostly have RCS capability.