Ghost Arks are transport vehicles used by the Necrontyr and the Necrons. They started out as wooden, space-horse-drawn carts for carrying the bodies of the dead into their tombs (think of that Monty Python "bring out your dead" skit).
After the Necrontyr leaders made their pact with the C'tan, they (by now motorized) took a whole new grimdark role. Most of the Necrontyr commoners didn't want to become soulless robots, so the Ghost Arks' crews would beat unwilling biotransferees half to death and dump their tied up, broken bodies in the Ark and send them away. After a while, the Ghost Arks were not crewed by the living but by the first Necron Warriors.
After the biotransference was done, the Ghost Arks were re-purposed once again to be the transport for warriors and to act as mobile repair stations. They can replenish and repair Warriors that are too damaged to fix themselves, though this brings a slight risk of overload to the Ark itself.
Aesthetically, the Ark looks a bit like a pirate ship and fires broadsides like one (well, with gauss flayers instead of cannons) too. It also has the usual Necron aesthetic of ancient Egyptian and vaguely skeletal design elements, but in an "odd coincidence", the "ribs" of the ship look a bit like the inverted Omegas of the Ultramarines.
A pretty interesting career for a millions of year old design of a wooden wagon, isn't it? Yeah i need a drink now.
The Ghost Ark was added to the Necron army list in their fifth edition Codex to serve as a dedicated transport. It shares the same model kit as the Doomsday Ark, except turned upside down and transporting your dudes instead of a giant rape cannon. And there are in fact 10 Warrior models aboard the thing, so unlike some transports it does have the cosmetic factor.
As it was a AV13, 13, 11 until it suffered a pen, it could transport a single standard squad of ten Warriors and repair them with the slight chance of taking a Hull point. But with the 7th Edition living metal and 4 hull points you wouldn't miss out on that.
9th Edition allowed it to double down on its main purpose : getting Warriors to the frontline and supporting them. It still has the same handy transport capacity and broadside of Gauss Flayer fire, but the current edition makes this a lot more valuable. In addition to being able to drop them close enough to the frontline to make full Gauss Reaper squads very effective as objective scoring shock troops, it's able to support the boys well once it's there by reviving D3 of them per turn, stacking with other sources. It also synergizes well with several Flayer-boosting dynasties.