Membership sites are those that require a subscription fee in order to have access to private content which is not made available to the general public.
While this type of site may not be right for all kinds of businesses, there are a number of businesses that can greatly benefit from setting up a recurring membership plan. Membership sites charge a fee which can be payable weekly, monthly, yearly etc. set up through automatic payments via your ecommerce options. If your product is service or training-related then you could benefit from establishing a membership-based online business.
Not only will you generate reliable monthly revenue but you will also be nurturing valuable long-term relationships with new and existing clients. The stronger your relationships the easier it is to sell your product(s).
Some types of membership sites include Health and Fitness, Dating, Coaching and Mentoring, Business Development, Marketing, and Investment sites as well as sites that are intended to act as a resource for a variety of professionals. If you think your business is viable as a membership site, it is well worth your effort to set up a paid membership component to your ecommerce.
Here is a link to a series of articles on how to start building your own membership-based online business: http://www.entrepreneurs-journey.com/704/how-to-launch-a-membership-site-part-1-build-your-preeminence/