Federation freighters, like Federation merchant ships, were specialized starships utilized by the Federation for hauling freight. These vessels encompassed a variety of designs, and were often operated with little or no crews.
Freighters used by the Federation in the 2260s remained limited to the speed of warp 2. (TOS: "Friday's Child") During the 23rd century, the Federation was known to operate several automated designs, including robot ore freighters and grain ships. These vessels were essentially flying cargo holds that possessed no life support systems. (TOS: "The Ultimate Computer"; TAS: "More Tribbles, More Troubles")
Many times, these vessels travel in convoys, which sometimes allowed them to be more or less susceptible to alien attack. (TOS: "Errand of Mercy") In those cases, they may be accompanied by an escort. In 2374, the USS Defiant assisted in escorting and protecting Convoy PQ-1 to the Vegan system during the Dominion War. (DS9: "Image in the Sand", "The Sound of Her Voice")