Freighters are mercantile ships that carry cargo between planets or stars. They may be anything from a fifty or a hundred meters to a kilometer or more in length. Smaller ones may be trans-atmospheric (these are more sophisticated and expensive) but it is more usual that they are vacuum only (larger and cheaper to buy, run and maintain).
They may be in-system only, ftl/jump only (docking near jump points) or both in-system and ftl/jump-capable ship. In-system and in-system/ftl freighters have a fusion torch or fusion pulse, and generally slow (a few may be moderate) acceleration. All have high delta V.
Three types of freighters are corporate, freehauler (often tramp), or military (often attached to fleets)
Corporate – these are generally modern, well maintained, often very large, and are easily able to compete with and undercut small operators. Main spaceways are served by corporates.
Freehauler – a freighter, especially a tramp spacer or similar sturdy second or third hand vessel, that is owned by a crew co-op rather than a shipping company or owner-captain. Freehaulers may come about through mutiny, co-op purchase, or when an owner-captain, unable to afford wages, pays the crew in ship shares and end up losing the ship from under them. Freehaulers are generally tramp spacers, servicing out-of-the-way worlds it is not profitable for the corporates to visit. Some may also carry passengers.
Military freighters are sometimes called “transports”