A vSphere ESX agent is a virtual machine and an optional vSphere Installation Bundle (VIB) that extends the functions of an ESX host to provide additional services that a vSphere solution requires.

For example, a solution might require a particular network filter or firewall configuration to function. A solution can use an ESX agent to connect to the vSphere Hypervisor and extend the host with functions specific to that solution. For example, the ESX agent can filter network traffic, act as a firewall, or gather other information about the virtual machines on the host.

ESX agent virtual machines are similar to services in Windows or Linux. They start when the operating system starts and they stop when it shuts down. The behavior of ESX agent virtual machines is transparent to the user. ESX agents are part of the hypervisor in the sense that vSphere does not consider a host to be ready until ESXi has started and all ESX agent virtual machines have been provisioned and powered on.

To integrate an agent with vSphere ESX Agent Manager and extend the capabilities of an ESX server, an ESX agent must be packaged as an OVF or a VIB module.

ESX agencies act as containers for ESX agents. ESX agencies aggregate information about the agents that they manage. For example, ESX agencies provide an overview of the the ESX agents that they contain by aggregating all the issues that relate to the ESX agents.