In my experience... 1) people get irritated if you call it TMJ instead of TMJ disorder (I'm not one of them). I was told once that "calling it TMJ would be like calling a torn meniscus a knee."
More seriously, mine is caused because I clench my teeth at night pretty severely, I sleep on my stomach with my head turned so it places a lot of pressure on my TMJ joints, and I found myself with my teeth (molars) touching nearly all day every day. I would get very severe headaches and feel sick over it. Things like chewing gum and eating tough to chew foods (overcooked steak, french bread, etc.) can cause it to flare up as well.
As far as treatment, I went to a chiropractor for a while and she could get it adjusted, but it would pretty quickly get out of alignment. I ended up being recommended to a specialty dentist who deals almost exclusively with TMJ disorder. He gave me retainers that I wear at night, and I wore the bottom one for six months during the day. At first, the bottom one had a ridge above my back teeth to keep my upper and lower teeth separated, but that was ground down so that it's less extreme of a gap now.
I wouldn't say it's been totally remedied for me, but the retainers helped immensely (as they should have for $4k!). I still clench at night, but the retainers trained my jaw to stay apart so I don't get the mid-day flare ups like I used to. Sometimes if I chew gum my jaw will feel tired, but rarely to the level of headaches and feeling sick.
Hope this helps! It's a lot of info