vSphere Auto Deploy lets you provision hundreds of physical hosts with ESXi software.

Using Auto Deploy, experienced system administrators can manage large deployments efficiently. Hosts are network-booted from a central Auto Deploy server. Optionally, hosts are configured with a host profile of a reference host. The host profile can be set up to prompt the user for input. After boot up and configuration complete, the hosts are managed by vCenter Server just like other ESXi hosts.

Auto Deploy can also be used for stateless caching or stateful installs.
Important: Auto Deploy requires a secure separation between the production network and the management or deployment networks as discussed in vSphere Auto Deploy Security Considerations. Using Auto Deploy without this separation is insecure.
Stateless caching
By default, Auto Deploy does not store ESXi configuration or state on the host disk. Instead, an image profile defines the image that the host is provisioned with, and other host attributes are managed through host profiles. A host that uses Auto Deploy for stateless caching still needs to connect to the Auto Deploy server and the vCenter Server.
Stateful installs
You can provision a host with Auto Deploy and set up the host to store the image to disk. On subsequent boots, the host boots from disk.