Solid wood is a board cut directly from a log, while hardboard is a composite material. I may be wrong on this, but I think hardboard is made from sawdust type material. Some one else may know for sure.
Hard board is fine for wall panelling, if you are going to paint or paper it after it's up. Solid wood is much more attractive if you want to make furniture.
Hardboard is also called Masonite. It is a man made product compressed under pressure. High density fiberboard is another name. A very hard surface. Medium density also call MDF is less dense and used for cabs, and other things.
Solid wood is just that like Gerry said, cut from a tree and in its original form.
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