It's Steampunk – with internal combustion.
But from there, you extrapolate what effect that tech level has on your society.
The tech in Steampunk is more often a new thing, or if not new, something wondrous that few people have access to, while in Dieselpunk, the tech is frequently ever-present, and the average person is likely to have ridden on some form of mass transit at some point in their lives. The technology is no longer new, and mass production increases accessibility. You have trams and subways, trains, and other public transportation.
You can have cars and motorcycles, and snowmobiles, and jetskis ("Waterworld") if you want. The horse has been, or is in the process of being replaced in all but the most, shall we say, traditional communities.
That last one's pretty huge. The horse has been around as transportation for a long time - thousands of years longer than mechanical vehicles, and that shift is a major turning point in history.
And of course, the weaponry.
You're looking at WWI and WWII level tech, with planes, tanks and machine guns. They took all that innovation that inspires Steampunk, and turn it into mechanized death. Machine guns were one of the reasons the first great war was so bloody. People suddenly have more ways of killing larger numbers of people faster than ever before, and this tends to mean that Dieselpunk is less often about discovery and invention and more often about social struggle. Even in video games.
Post Apocalyptic stuff is often Dieselpunk, and I can tell you why it usually ends up Dieselpunk and not Steampunk. It's because why would we get thrown back to steam level tech, if we had internal combustion? If there's a loss of tech, it's likely to go back to the last level where the average person had access to the technology and could do routine maintenance on it, and find someone who can fix it if it breaks down.
It's really wide open. I wouldn't dare try to define it, only throw out of couple of ideas to start you off.
Military is a popular motif – uniforms from the WWI to WWII period show up at cons. Army boots and dog tags are great accessories. The era saw the birth of the classic pinup girl, and victory rolls. Short hair on women became more common. Cuban stockings, and bright colours all around.
But then there's the post apocalypse aspect that opens it up wide to where anything goes. If you're going the post apocalypse route, feel free to go grungy, with lots of rips. Fishnet is a go.
One thing I notice is the style of goggles is very different.
The Steampunk goggles are generally round lenses, the sort that might have attachments for magnifying things like clockwork gears, or for welding, perhaps. Dieselpunk goggles are generally driving goggles, to protect one's eyes from the wind when driving a motorcycle or car, or aviator goggles, for flying. They have a bigger lens, and more side visibility. You gotta be able to see them bandits when they're flying up from your six o'clock, huh?
On top of the tech level, there's also the vibe of the era it represents. Part of the appeal of Steampunk is the romanticism of the Victorian era. Dieselpunk, though, we're looking at an era of social upheaval. The world was faced with two world wars, and entire countries had to choose sides and fight. Young men coming home from war came home to their jobs having been filled by women, and their women demanding the vote. People of colour, too, demanding to be treated the same as everyone else.
Huge shifts in social responsibility and equality. It was a time when people weren't yet jaded by the lies told by politicians, and and it was worth fighting for a cause you believed in. People still believed that they could make a difference in the world.
That's why I think Dieselpunk fiction and movies tend to be ones that delve into those sorts of themes. It's what draws me to it, and why I write it, myself. I hope it will be what draws people to my writing.
--Lindsay Kitson, The Dieselpunkette