sounds like pre-finished gypsum board. It kinda like drywall that has had a vinyl covering bonded to it...almost like wallpaper. You commonly see this material used in manufactured houses such as singLE and double wide mobile homes.
I have joined and finished this stuff many times...it's easy.
1. Wash down the wall with a good detergent soultion.
2. Apply a coat of latex primer along the joint area about 3 inches on each side.
3. Use the self-adhesive TYPE fiberglass mesh drywall tape to tape the joint and apply a thin coat of drywall joint compound with a 4 inch knife.
4. Allow the joint to dry thoroughly the apply a second coat and then a final skim coat of drywall compound, using a larger knife each time. You may need to sand lightly between coats.
5. When thoroughly dry, sand the joints smooth.
6. Apply AT LEAST 3 coats of latex primer to the entire wall [including the joints].