Voluntary service is a popular social service activity in modern society. It is a democratic movement motivated by self-initiative and self-help desire. People contribute their time, money and talents without asking for material reward in order to help others in need and to improve their community life. The main theme of this paper is to discuss the various definitions of volunteerism, voluntary organization, quasi-public organization, and the International Classification of Non-Profit Organizations. The mixed nature of voluntary organizations and non-governmental organizations is briefly examined. The philosophical principles of volunteerism and legal definitions of voluntary organizations are discussed. The scope and contents of voluntary services are described in details based upon the I.R.S. classification of non-profit organizations in tax codes. The rationales for the existence of non-profit organizations and voluntary services in modern society are discussed. The historical development of voluntary services is analyzed in relation to the social-political, social-economic policies, and community services in American history. The paper concludes with an emphasis of important leadership training for voluntary services and the future trend of voluntary organizations in modern society.