Perhaps the simplest definition would be to call it your volunteer work for God.

We can use words like service or duty, but what we're really talking about is our desire and ability to work with God to get the Good News of Jesus out into the world. If that means giving Him the best of our time, talents, and treasures, letting him employ us for His work, then so be it.

At FUMC we often call our departments, or programs, ministries; i.e. the Children's Ministry is the department that encompasses all of our Children's programs.