The Microsoft Message Queue Server (MSMQ) uses a site/enterprise model. Typically a site is a physical location, such as a building. An enterprise consists of one or more sites and typically represents an organization.
At the heart of MSMQ is the message queue information service(MQIS) database, which runs on top of SQL Server. An enterprise has a single "master" MQIS, called the Primary Enterprise Controller. Each site has its own MQIS, called a Primary Site Controller and zero or more Backup Site Controllers . Finally, there are the individual client machines, each of which has its own queue manager , implemented as a service. The Primary Enterprise Controller can also be a Primary Site Controller, and any controller can also be a client.
Message queues can be either public or private. Public queues are stored in an MQIS database and are accessible across the network. Messages in a public queue are routed throughout the enterprise, under the control of MSMQ. Client application messages move from the client's queue manager to the destination queue by traveling between the queue managers of the site controllers.
Private queues are maintained by the local queue manager and are not registered in the MQIS database The scope of private queue messages is limited to the machine they are on.