In general, a service is an action performed as an (intended) benefit to another individual person or to other persons, including other creatures, either voluntarily or as required by need.
In economics, a service is labor that can be bought or sold, as in professional advice. This is in contrast with a good. The entire market consists of goods and services.
At law, "service" refers to the delivery of notice of a lawsuit to the affected party, referred to as "service of process."
In psychology, self-serving persons act only to benefit themselves more than others, sometimes using others needs to serve their own selfish purposes, solely as a means of service to themselves alone for personal gain. Victims of such persons, after giving themselves in service to them, often come to realize that they have been "used".