Steampunk is a genre of science fiction that imagines what the world would be like if Victorian steam-powered technology persisted into the modern era. It also refers to a visual style commonly associated with the late 18th to early 19th century as applied in a modern context.
While I've never read any of the zombie-enhanced literature, I do appreciate the resurgence of interest in 19th-century, albeit with a darker twist.
At its core, steampunk represents a culture of anachronism that is part of the cyberpunk speculative fiction genre. I should note that this is similar to, but distinct from, the activities endorsed by the Society for Creative Anachronism which targets pre-17th century Europe.