Component Object Model is a Windows binary interface for inter-process communication. Communication is performed in a form of client-server interaction - a client can call methods provided by a COM object (acting as a server) by referencing the COM object by pointer or reference.
Applications can implement COM interface in multiple ways - the bottom line is that the binary (compiled) object must provide methods for other processes to interact with. Such methods can be used for anything - for example, MS Office applications serve interfaces (APIs) for document creation, manipulation etc.
COM classes can be identified by CLSID or name. COM object is of course an instance of specific COM class.