As a Noun:
1. Someone who does work, especially socially useful work, for which they are not paid. English = volunteer.
2. Someone who offers to do something because they choose to, not because they were forced to do it. English = volunteer.
As a Verb or Adjective:
1. Of a person, to choose or to offer to do something without being forced to do it. English = volunteer.
2. Of an action, to be done because you want to do it and not because you have been forced to do it. English = (be) voluntary.
3. Of a person, to formally ask to be allowed to have something or do something, especially to have a job. English = apply.