The definition of volunteer is difficult; because people use this for different meanings. In Japan this word is used with 'katakana' "Borantia" for distinguishing from the traditional "Kinrou Houshi (unpaid labor service)". In Japanese case a volunteer has the character of autonomous voluntary individualism and democratic equality.
I will define the volunteer as the social activity of individuals with unpaid voluntary work. In this treatise I have picked up the volunteer as a work.
Generally speaking, "employees work for money and volunteers for love". Then the volunteer's appreciation of his work is different from employee. Employee's purpose is wage and his labor is estimated from the point of abstract simple human activity which can be measured by the time for wage. But volunteer's purpose is the giving for unhappy people and his work is estimated from the point of concrete useful activity.
The work of volunteer is unpaid. Sometimes this fact is criticized that it's only cheap repair or stopgap of capitalism's contradiction and defect. To respond to this critique volunteers must too struggle politically against the evil of capitalism or each government. But volunteer is not only the repair but also the pioneer who discovers and leads new useful social activity.
The work of volunteers is done at their free surplus times and is free, voluntary, agreeable and full activity. If their work is uncomfortable or empty, they can withdraw from it, for their work is unpaid. If they feel disagreeableness or compulsion, then they may stop their work, for no wage.