An in-law is someone who is a relative because of marriage, like your husband's sister or your wife's father. You can refer to your spouse's entire family as your in-laws.
In some countries, a married woman moves in with her in-laws, symbolically becoming part of their family. In the U.S. some families live with extended family members — a man may live with his wife, kids, and his mother- and father-in-law — but more often each individual nuclear family lives separately. In-law originally meant "anyone of a relationship not natural" or "not by blood."