I've seen a couple of questions recently about the differences between Agent managed and Agentless assets, so I thought I'd explain the differences and the relative merits.
You can manage operating systems and virtualization technologies in one of two ways - by installing an Ops Center Agent on them, or by providing Ops Center with credentials that it can use to reach the asset. The agent is tailored to the system - there are separate types of Agents for Zones and LDoms.
If you don't want to have anything else installed on a system, agentless management can provide management and monitoring capabilities. However, there are some features that aren't available for assets that are managed agentlessly. There's a table in the OS Management chapter that explains what features are and are not available with agentless assets.