Explanation/Reference:
* The Client Tier represents access to management content and operations as well as end users accessing the appropriate business solution. Administrators utilize a browser based console to perform their management tasks using a standard browser interface.
The management console which is lightweight, easy to access and firewall friendly, enables administrators to centrally manage their entire environment.
* The Management Tier renders the content and interface for the management console that gives access to management operations such as monitoring, administration, configuration, central policy setting, and security. The Management Tier controls the accessing and uploading of management information.
* The Managed Target Tier contains the named infrastructure components that are required to be managed and monitored. It is common to utilize a combination of agent based and gateway (a.k.a. proxy) patterns to monitor and manage hosted and non-hosted targets.

Reference: Oracle Reference Architecture, Management and Monitoring, Release 3.0