A Federation tug was a class of small specialized tug starships used exclusively for towing damaged or disabled vessels. These ships were used by the Federation and operated by Starfleet in the 24th century. Federation starbases were equipped with these types of tow ships. (TNG: "Phantasms")
In 2370, when the warp and impulse drives of the Enterprise-D were disabled by interphasic organisms, Vice Admiral Nakamura suggested he might send a tow ship to bring the Enterprise-D to Starbase 84. Captain Jean-Luc Picard declined the offer. (TNG: "Phantasms")
During the Dominion War, tugs were utilized in the evacuation of disabled vessels from battle zones. The USS Fredrickson was one such starship to benefit from these utility vessels in 2374. (DS9: "A Time to Stand")
According to a comparison chart drawn up by Gary Hutzel and Larry Nemecek, the length of the Federation tug, which they had listed under the name "tug boat", was 216.5 feet in length. . Its appearance in "A Time to Stand" makes it stand out as one of the few utility vessels, apart from the shuttles and cargo management units, of Starfleet ever to be shown on screen.
The episode "A Time to Stand" needed a shot of a retreating flotilla of Starfleet vessels. To beef out the scene, the production staff built several new ships, kitbashing them out of parts from commercially available AMT/Ertl-Star Trek model kits. Amongst them was the Federation tug, built by Judy Elkins, and affectionately called the "USS Ertl" among the staff themselves. (X)  The warp engines were those of a D'deridex-class Romulan warbird, originating from AMTkit no. 6858. Though these ships were mentioned before in dialogue in Star Trek: The Next Generation, "A Time to Stand" marked the only on screen appearance.