It is a by-product of the wood industry. Usually used externally, though some have taken it internally. MSM is a derivative or variation of DMSO. It has long been used for animals (especially horses) to heal sprains, etc. My uncle has used it for years for his Arthritis (after he found that it worked so well for his horses.)
Some people are afraid of it, as it is a solvent. The only precaution is that you don't want any contaminates on your skin, as the DMSO will drive it inside you. So, one needs to have clean skin. Your breath will smell somewhat like garlic after you use it. It's the sulfur.
Apparently it drives oxygen and whatever substance you use with it, into your body. It's like an instant healing. As some have said, it doesn't cure anything, but it stops it until you can figure out what to do. It seems to instantly stop inflammation!