Karma Yoga means Selfless Action. To be a Karma Yogi (Volunteer) means to be in harmony with the existence. A true Karma Yogi is someone who can perform every action with a sense of devotion without being attached to its results. For a Karma Yogi, Life becomes a Leela, a Divine Play, so effortless and ecstatic.
Karma Yoga is not a service to somebody but, an action done with a sense of complete responsibility towards the whole existence.