What is the difference between single threaded and multi threaded process?

A write-up of the answer to this question can be found here:

Microsoft.com/en-us/library/ms693344(VS.85">http://msdn.Microsoft.com/en-us/library/ms693344(VS.85).aspx

"There are two types of apartments: single-threaded apartments, and multithreaded apartments. * Single-threaded Apartments-Single-threaded apartments consist of exactly one thread, so all COM objects that live in a single-threaded apartment can receive method calls only from the one thread that belongs to that apartment. All method calls to a COM object in a single-threaded apartment are synchronized with the Windows message queue for the single-threaded apartment's thread. A process with a single thread of execution is simply a special case of this model.

* Multithreaded Apartments-Multithreaded apartments consist of one or more threads, so all COM objects that live in an multithreaded apartment can receive method calls directly from any of the threads that belong to the multithreaded apartment. Threads in a multithreaded apartment use a model called free-threading. Calls to COM objects in a multithreaded apartment are synchronized by the objects themselves."