Have you ever volunteered? If you've ever done something for someone else without receiving payment for it, then, yes, you've volunteered!
Not always! Some people simply feel the urge to promote good causes or help improve the lives of others. Such people are altruistic, which means they do things selflessly, with no thought for their own benefit.
Some people volunteer to gain skills they might need in the future. For example, if you want to be a doctor or a nurse some day, you could volunteer at a hospital and learn more about the working lives of doctors and nurses.
Others volunteer to meet new people with similar interests. For example, volunteering to help a local church group build a house for someone less fortunate might allow you to meet other nice people who might one day become good friends.
People from all walks of life volunteer their time in many different ways. Sometimes people volunteer in areas they work in, such as hospitals (medicine) or schools (education). Others choose to volunteer in response to specific events, such as natural disasters like tornadoes or floods.
If your school has a parent-teacher organization (PTO), then you have a solid group of volunteers working hard to make sure your school is the best it can be. When you see people helping out at school, make sure to thank them!