War World was the planetary seat of Mongul's empire.
The planet's original name is unknown. Mongul named it "War World" for the gladiatorial spectacles he continuously staged on the planet, for the amusement (and distraction) of his people. It was also the seat of his government (though he paid little attention to the daily business of ruling his subjects) and the headquarters of his military.
Outside of the massive arena where the fights were staged, the cities of War World were run down, and its citizens largely destitute and downtrodden.
In the comics, War World is a space-faring battle station the size of a small moon or planet (similar to the Death Star from Star Wars), which Mongul uses to travel the galaxy and conquer various star systems. In the Gladiator story arc, Superman successfully defeats Mongul and War World is destroyed.
In the Return of Superman storyline, Mongul, in alliance with The Cyborg (Hank Henshaw) comes to Earth and destroys Coast City (the home of Green Lantern Hal Jordan) then builds a massive city-sized engine, as part of his scheme to transform Earth into another War World.