Membership objects are collections of roles that define special types of membership for website users. They are typically assigned to users for a limited period of time, often as part of a purchased product.
More information can be found in the Developer's Guide -> Development -> Membership, permissions and security -> Memberships chapter.
Page url: http://devnet.kentico.com/docs/6_0/contexthelp/index.html?onlinemarketing_contact_membership.htm