The class library in Borland Delphi is known as the
Visual Component Library or
VCL. Delphi’s class library is written in Delphi, and
source is shipped with Delphi. This class library is highly regarded as being simple to use and flexible. It and does a good job of hiding the details of the
Windows API.
“The Visual Component Library (VCL) is a hierarchy of classes—written in Object Pascal and tied to the Delphi IDE—that allows you to develop applications quickly.” – Delphi help
All objects descend from TObject by definition, all objects that you can drop on a form descend from TComponent, and anything that wraps a windows control descends from TWinControl.
“Despite its name, the VCL consists mostly of nonvisual objects.” – Delphi help.
Many classes in the VCL have non-visual roles, for e.g. Exception claase based at Exception list objects like TStringList and TObjectList and Operating System objects like TCriticalSection and the longest-named of them all: TMultiReadExclusiveWriteSynchronizer
You may have noticed that it is convention for all Delphi class names to begin with T for type, with the exception of Exceptions.
The thing that I don't like is that is not open source - even when you find a bug and know how to fix it, convincing Borland of that is a huge pain in the rear.