The virtual data center (VDC) is a combination your cloud resources (virtual CPUs, memory, drive space, networks, and so on) accessible within one account. This is an environment, where you create virtual machines, containers, networks and so on. You can increase its capacity as required by purchasing necessary resources (CPUs, memory or drives).
vApp is a container that hosts virtual machines. You can create a empty container or a container with virtual machines. Containers enable you to manage a group of virtual machines as one unit. This is especially useful if you manage a large virtual infrastructure. You can create templates on the basis of vApp. If you have the needs to create the groups of virtual machines of the same type regularly, this vApp facility will save you time. vCloud Director also enables to create a separate vApp network to prodive additional insulation between two vApps. vApp can be used to group virtual machines relative to the same business application.
Virtual machines (VMs) can be created from templates in the catalog (together with the OS) or from the scratch, installing the required OS from an uploaded template or image.
Catalogs are folders to store vApp templates, virtual machines and media files.
Org VDC Networks are the networks of your virtual data center, available by default for all vApps. Org VDC networks can be isolated (without access to the internet) and routed (with access to the internet).
In addition to Org VDC networks, available for all vApps, you can create vApp networks. Only virtual machines of correspondent vApp will communicate in this network: it won't be accessible for virtual machines from other vApps. The vApp network can be connected to the Org VDC network, allowing vApp to communicate with other vApps of the organization or external vApps.