VOMS stands for Virtual Organization Management Service. It manages membership lists and rols for a virtual organization (VO). For example, a group of twenty scientists across three universities might collaborate with each other for a long-lived experiment. In order to keep track of who is in their virtual organization, they use VOMS. VOMS can keep track of each user, and their rols. For example, the principal investigator might have the role of "TechLead", while others do not. Other grid software such as EDG mkgridmap and GUMS can consult VOMS to decide if a user is part of a virtual organization.
Users are described by their X509 certificate's Distinguished Name, so all users must have X509 certificates.
VOMS in the VDT is composed of two programs: VOMS-Admin is a front-end web application and VOMS is the back-end database that tracks users.