The ancient city that was once a capital of the Great Old Ones society called The Imperium, was destroyed by Blaine the Mono while leaving with Roland Deschain and his Ka-tet by releasing NBC weapon stockpiles from beneath the city.
Lud only appears in The Waste Lands. Roland and his ka-tet travel there following the path of the beam. It is also mentioned in the book Rose Madder, another work of Stephen King's.
The Pubes were the original inhabitants of Lud and were scientists. Lud was one of the last bastions of science in Roland's world. It became a prime target for harriers. Now, the Pubes have a highly ritualized society. They kill one of their own whenever the "God-drums" start playing. Eddie identifies the god-drums as being the drum track of "Velcro Fly", a song by ZZ Top.
The Grays were originally the army of the famous harrier, David Quick. Now, they are led by his great grandson, Andrew Quick, aka the Tick-Tock Man. Their base is located in the underground sewer system.