VOB is short for Versioned Object Base.
It's the file repository of ClearCase. You can have many VOBs in a ClearCase site. The best comparison is really if you look upon ClearCase as a file system. The the VOBs would be the equivalent of a file system partition.
In UCM you have special VOBs called Project VOBs or just PVOBs. These VOB's (typically) doesn't hold any files or version, only metadata about the ClearCase objects.
VOBs can have AdminVOBs. An admin VOB is not really describing a special type of VOBs but more a realtion between two VOBs.
If a VOB has a hyperlink called
When a object is created internally in ClearCase the type (class) is sought internally in the current VOB - if no such type is found, then seach will continue in the chain of AdminVOBs.
Project VOBs (PVOBs) always paly the role as AdminVOBs (common namespace) for all the VOBs that hosts any components defined in that PVOB. Sometimes the VOBs hosting the components are refereed to as client VOBs.