how can i describe an organization that may contribute to my professional knowledge?

Answer 1

This depends to a large degree upon the depth to which you want to go in describing the organization. However, at the core, I think a professional organization does several things.
a. they give contact with others in the same profession.
b. they provide continuing education.
c. they provide a means of publicizing new/creative/different ideas.
This is not an exhaustive list but it should get you started.

Answer 2

describe it in terms of growth, development of a varied experience, and good feedback from mentors.

Answer 3

Describe the organization's goals and how it helps its members learn more about the field.