At this point in time there isn't a definitive answer on what doms are, although muscular micro tears seems to be the most commonly held belief.
I find DOMS useful if I am trying out a new exercise. Usually I am going a bit lighter and more reps to feel out the movement so this accumulated volume usually results in muscle soreness the next day. Where I get sore is a rough (very rough) estimate on what muscles got worked.
This can also be a bit false however, as certain muscles on my body very rarely get DOMS.
Once my body has gotten used to a certain movement, DOMS reduce and/or disappear. If I'm still getting stronger but no soreness, the exercise is still working.