Decopunk is a recent subset of the Steampunk genre, centered around the Art Deco and Streamline Moderne art styles, and based around the period between the 20s and50s.
In an interview at CoyoteCon, steampunk author Sara M. Harvey made the distinctions - "...shiner than DieselPunk, more like DecoPunk." and "DieselPunk is a gritty version of Steampunk set in the 1920s-1950s. The big war eras, specifically.
DecoPunk is the sleek, shiny very Art Deco version; same time period, but everything is chrome!"
DecoPunk is more of a brighter future, as if the depression and wars never happened. It's a bit pulp, like worlds of the early Superman, the Rocketeer, the Phantom, and the Flash. It's all hopes and dreams.
The term was also independently proposed for the movie Sky Captain and the World of Tomorrow and the computer game Bioshock, though the latter is set in an alternate history 1960 placing it outside the usual time period associated with decopunk.