A service was labor provided by an individual or establishment either in exchange for money or voluntarily.
In 2370, Major Kira Nerys and Chief Miles O'Brien infiltrated the Hutet labor camp on Cardassia IV by bribing a Cardassian overseer with an offer to use Kira's "services" for just two strips of latinum. When the overseer lowered the force field protecting the camp, he was attacked and subdued by the two. (DS9: "The Homecoming")