Freighters are the biggest ships in No Man’s Sky. They are also called capital ships. You can only have one at a time. They have the same classes as other starships and their inventory can be upgraded via specialized modules you can scavenge in the game. There is also a way to get an s-class freighter at the beginning of the game for free.
All you have to do is make a manual save prior to warping to a new star system and defending a freighter which will be awarded to you as a part of the quest. Keep reloading the save until you get lucky and an s-class freighter spawns. You can save hundreds of millions units this way, if you have the patience to do it.
Freighters also serve as portable bases and they are highly expandable. You can build insanely huge interiors inside them and then use them as you see fit. You can also expand them in terms of their maximum number of floors. But be careful, expanding them too much might lead to FPS drops and weird lighting issues.
Other than that, freighters can be upgraded in terms of technology (it’s researched inside it) and they will allow you to assemble a fleet of frigate ships to send on missions (you have to buy them from other freighters). Once a mission is complete, they will return units and other valuable items/materials.
Freighters come in all shapes and sizes but their interiors are all the same, with the exception of having different colors on the inside (some are blue, others green, others red etc).