what do worms breathe, and how?

Answer 1

If you mean earthworms the breath through their entire skin, by oxygen passively diffusing across the epidermis directly into the tissues. (They have so lungs.) This means their skin is also easily dried out.
http://edis.ifas.ufl.edu/in047

Answer 2

If you are talking about land dwelling worms, they breathe the same air we breath. They breathe through spiracles (little openings) down the sides of their bodies, which sends air directly to the different parts of the body or segments.