"WT" = "Windchill Technology" , or so I've been told. I always expected "Windchilll" to be abbreviated "WC" as in "WCPart", but I suspect that "WC" looks too much like "Water Closet" so they used "WT" instead.
WTObject is a generic, persistable object of some kind that all business objects extend from. WTParts and WTDocuments are all WTObjects. Parts are business objects representing components and assemblies in a product. Documents are containers with metadata to hold files, like PDF and Word files.
Somewhere in the PTC Online Documentation, I think you can still download the Windchill 9 Object Model in UML. You should try to find that since it shows all of the basic Windchill relationships using the unified modeling language as it used to come out of Rational Rose back when PTC development was still using that tool.
Al