From the ever-useful Wikipedia:
"Leonardo da Vinci first proposed the idea of a steam-powered cannon that would launch a projectile using only heat and water. Some sources say Archimedes was the original source of the idea.
The device would consist of a large metal tube, preferably copper as it is the best conductor of heat, which would be placed in a furnace. One end of the tube would be capped and the other loaded with a projectile. Once the tube reached a high enough temperature, a small amount of water would be injected in behind the projectile. In theory, Leonardo da Vinci believed, the water would rapidly expand into vapour, blasting the projectile out the front of the barrel."
The Holman Mk II Projector, used in the early part of World War II, used steam to fire a small projectile up to 600 feet. The Royal British Navy contracted Holmans (a maker of machine tools) to build the Projector after seeing it tested. Perhaps the most interesting thing about the weapon was that it left a small cloud of black smoke after it fired. This had the unexpected, but welcome, consequence that the Luftwaffe believed many British vessels were more heavily armed than they actually were.
Now, the writer of steampunk has a lot of freedom if they decide to use steam cannons. You can attach a boiler and pistons to the rear, mount it on a tank, ship or zeppelin, or even have cannons like that be so rare the only place they are is around government buildings.
In terms of design description, all you really have to say is "steam cannon" and you're pretty much set. "Steam-powered cannon" if you want to get really technical of course.
Another post will deal with why you might want to use certain steampunk items, such as steam cannons, when there are perfectly serviceable real-life alternatives.