Template: Virtual Machine definition. These definitions are managed with the onetemplate command.
Image: Virtual Machine disk image, created and managed with the oneimage command.
Virtual Machine: Instantiated Template. A Virtual Machine represents one life-cycle, and several Virtual Machines can be created from a single Template. Check out the VM management guide.
Virtual Network: A group of IP leases that VMs can use to automatically obtain IP addresses. See the Networking subsystem.
Virtual Data Center (VDC): Defines an assignment of one or several Groups to a pool of Physical Resources. Typically this pool of Physical Resources consists of resources from one or several Clusters that could belong to different Zones or public external clouds for hybrid cloud computing.
Zone: A group of interconnected physical hosts with hypervisors controlled by the same OpenNebula.