자원(하다) means to volunteer, and 봉사(하다) to serve (for good purpose). You can understand the terms as follows. * 자원봉사자 - a volunteer. Most common word for people who voluntarily do work without compensation. * 봉사자 - people who work without compensation. Similar to 자원봉사자, but with less emphasis on volunteering. * 자원자 - someone who volunteers for anything, not limited to serving or any other particular activity. 자원봉사자 is a widely used word, while 봉사자 and 자원자 are less so and more context-dependent. There is another popular word, 도우미 (a noun derived from 돕다, to help), which has the senses of 봉사자 and 자원봉사자.

First, 자원자 is who volunteer something by own will with or without payment because s/he wants to do it. Second, 봉사자 is who serves the organization, society, and nation like NGO. Last, 자원봉사자 mostly means who serves for nothing, especially national event of Korea such as Olympic, Worldcup, G20 submit conference. For example, recently held Pyeongchang Winter Olympic had a lot of 자원봉사자. I hope you can understand different meanings between those word in daily life.