Single Threading: This is the simplest and most common threading model where a single thread corresponds to your entire application's process.
Apartment Threading (STA): This allows multiple threads to exist in a single application. In single threading apartment (STA), each thread is isolated in it's own apartment. The process may contain multiple threads (apartments) however when an object is created in a thread (i.e. apartment) it stays within that
apartment. If any communication needs to occur between different threads (i.e. different apartments) then we must marshal the first thread object to the second thread.
Free Threading: The most complex threading model. Unlike STA, threads are not confined to their own apartments. Multiple treads can make calls to the same methods and same components at the same time.
Shashi Ray