Good question! I can't really describe it but I know it when I see it.
In general terms, wallboard is drywall is drywall, ..., it's a paper and gypsum-based product. Just go to any Home Center and visit the Building Materials aisle looking for those 4' x 8' paper/cardboard-like sheets with the hard, solid plaster 'stuff' that's sandwiched in the middle. Traditional wallboard, as most folks know it, have a brother and a sister too as well as cousins. There's Blue Board and Green Board, and Cement Backerboard too. It can get reeeeeeal busy, you know.
I have to warn you, and plez don't misunderstand me. If you have to ask this question, you may want to get a book on how to do your particular project. THere are a lot of great Kitchen & Bath Books out there, some from Time-Life and Reader's Digest to name a couple. You don't want to have to do this job again a year or so later. It's gonna be hard enough and disruptive enough the first time around.