There are two words for a 'volunteer' in French: un bénévole ou volontaire.

What is the difference between the two?

A bénévole is a volunteer who offers his time outside of his normal job, often part time. He could be anyone, a student or a professional. He is normally unpaid, and receive minimum legal protections for his services.

A volontaire is a volunteer who offers her specific professional expertise for a certain period of time, often for an organisation and often paid for her work.