The basic difference between the two is that Active members work in VFX, and Associate members typically work with VFX. Here is the official definition from the VES Bylaws:
Active Membership – An Active Member is a member who has been actively engaged in the production or creation of Visual Effects for not less than five (5) years, within the last ten (10) years preceding application for membership. If the applicant seeks Active Membership, two (2) Active Members in good standing must endorse this application.
Associate Membership – An Associate Member is a member whose professional activities have involved a close relationship with, and exceptional achievements in, the field of Visual Effects for not less than five (5) years. If the applicant seeks Associate Membership, two (2) Members in good standing, who are either Associate or Active, must endorse this application.