A volunteer is someone who works for a community or for the benefit of environment primarily because they choose to do so. The word comes from Latin, and can be translated as "will" (as in doing something out of ones own free will). Many serve through a non-profit organization – sometimes referred to as formal volunteering, but a significant number also serve less formally, either individually or as part of a group.
A volunteer may or may not get paid or receive compensation for services rendered. General Andrew Jackson's Tennessee Volunteers were paid by the General personally, although he did seek reimbursement at Washington for his expenditures.
Volunteering comes in many forms: serving food at the local homeless shelter, providing computer technical support to a non-profit organization, acting in a leadership capacity on a charitable organization's board of directors or coordinating the emergency response in the case of a disaster. Around the world volunteer centers exist to support the voluntary sector and make a difference in the communities that they serve.