Weblogic Admin Server vs Managed Servers

Weblogic defines the roles of Admin/Managed server as follows:

• Admin Server
  • can be only one for a Domain, which works like a Central Configuration/Monitoring controller for the entire domain
  • runs the console, where you can create new managed servers, define DataSources (DB connections), JMS queues, deploy apps to Managed Servers, etc.
• Managed Server
  • additional WebLogic Server instances
  • you deploy Web applications, EJBs, and other resources onto the Managed Servers and use the Administration Server for configuration and management purposes only. This is where you run your apps.
  • Managed Servers can be grouped to clusters.
  • one of the examples of a Managed Server is a server running SOA applications, like OSB or BPEL.

See e.g. Understanding WebLogic Server Domains.

WebLogic Server Instance is a run time JVM and it is part of WebLogic Domain. First Server Instance is called as Administration Server and it is used for Administering the domain. Other Server Instances in a domain are called Managed Servers and they are used for hosting the deployed applications.