Ok smartass, not that kind of joint.

Joint compound is a substance that is used to cover the imperfections that are are visible when two pieces of sheetrock meet. The sheetrock does not make a perfectly flush seal. This being the case, something must be used to give the illusion of a seamless wall. That is where the joint compound comes into play.

The seam is made by slabbing on joint compound into the little gap between the two pieces of sheetrock. You apply the stuff with a knife. No, not a carving knife, or butter knife, or some such thing, but rather a flat blade that you use to "squish" the joint compound into the gap. You then take a wider flat blade and feather out the joint compound. Next step: put on seam tape, which is not actually tape, as it doesn't adhere on its own to anyhting. It is something like heavy paper. It usually has a crease down the middle of it for application on corners. It has almost the texture of paper money, now that I think about it. After applying the seam tape, you repeat the process with the joint compound, putting on a first application, and then feathering it out with progressively wider knives.

Once the joint compound dries, it very much resembles the outer layer of sheetrock ( it may be the same substance, I dont know). However, at this stage it is still fairly rough to the touch. That is okay, because you probably want to go over the process again, without the need to apply the seam tape though.

After the second application, and drying, the joint compound needs to be finished. You do this by sanding it down smooth. I recommend starting with 100 grit, then 150, then maybe 200 or even 250 grit. That gets it smooth as a baby's bottom.

What are the ingredients of joint compound? I'm not sure. I think it must have some chalk or silica in it, water, and many other assorted goodies. The stuff has the consistency of very gooey clay, kind of like pancake batter that is a bit too thick.

My wife, upon smelling it, said that it stank like a woman's bathroom where too many sanitary napkins, festering in the trash, had been stinking up the place. I'll have to take her word for it.

A final note. Really, the stuff is kinda cool, but definitely, shower after using it. It gets all over the damnplace, including the person applying it. And it itches when it dries.