Eg. You're in an association so you are helping persons willingly and you are looking for no reward.
closed as too broad by tchrist♦, Drew, Brian Hooper, Chenmunka, Edwin Ashworth Jan 2 '15 at 1:16
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"Volunteer" sounds like what you're describing.
Your work in the association can be considered "volunteer" work as Ryan already suggested, and it can be also considered "altruistic":
from the Free Dictionary:
- Unselfish concern for the welfare of others; selflessness.