unpaid work undertaken freely by individuals as a service to others
A volunteer is someone who helps a common cause and who does not expect to get paid for it. There are many cases, such as cleaning up after a disaster, where anyone can help. There are many cases, though, where the volunteers are trained specialists. Examples for these are the volunteer fire departments or a doctor or nurse working for organisations such as the Red Cross. Another example are the people guiding police dogs, which may be trained for emergency rescue.