well basically a kind of anachronistic au in which steam is still used to power all engines (think electricity never happened) and technology developed based on that

aesthetically speaking, artists usually stick to designs that revolve around steam machines (and gears/cog wheels but if you overdo that, it's not well-received lol) and fashion inspired by that time in which steam machines were big, i.e. roughly the Victorian age

a lot of its appeal is also thinking how steam-powered aviation would work, so there are lots of airship/airpirates stories and so on

hope this explains the gist of it ^.^v