Answer by US_DR_JD
The ANA is a professional organization which represents RNs and coordinates the professional needs of nurses. It would be comparable to the American Medical Association for MDs or the American Osteopathic Association for DOs.
A special interest group would be one that has a directed scope, such as the one which deals with gender or racial issues in nursing. An example is the National Association of HIspanic Nurses. While an excellent organization, they have a directed focus, as opposed to the general welfare of the profession.