Instance Manager: Used to startup and shutdown an Oracle database. Used to startup and shutdown an Oracle database.
Schema Manager: Used to create and manage tables, indexes, views, and other schema objects.
Security Manager: Used to grant system and object privileges to users, and to otherwise manage database security.
SQLPlus Worksheet: Used to execute ad-hoc SQL statements, and SQL scripts.
Storage Manager: Used to manage tablespaces and datafiles.
Oracle Management Server: The Management Server represents the middle-tier, and is where event monitoring logic and job submission logic are implemented. The Management Server also runs the web browser interface.
Repository Database: This is the database where OEM stores its information. It does not necessarily need to be physically on the same machine as the Oracle Management Server software.
Managed Database: Any number of remote databases may be managed using Enterprise Manager.
Oracle's Intelligent Agent has been deprecated since the release of Oracle 10g.
Oracle Management Agent (Management Agent)
The Management Agent is an integral software component that enables you to convert an unmanaged host to a managed host in the Enterprise Manager system.
The Management Agent works in conjunction with the plug-ins to monitor the targets running on that managed host.
Software runs on each database server, and is responsible for ensuring that scheduled jobs for that server are actually run. When you schedule a job from the console, the management server stores that information in the repository, and communicates it to the appropriate agent.
The agent ensures that the job gets run, and reports status back to the management server. That way, network outages do not prevent scheduled jobs from running.