COM clients communicate with objects through COM interfaces. Interfaces are groups of logically or semantically related routines which provide communication between a provider of a service (server object) and its clients. The standard way to depict a COM interface is as follows: 

For example, every COM object must implement the basic interface, IUnknown. Through a routine called QueryInterface in IUnknown, clients can request other interfaces implemented by the server. 

Objects can have multiple interfaces, where each interface implements a feature. An interface provides a way to convey to the client what service it provides, without providing implementation details of how or where the object provides this service. 

Key aspects of COM interfaces are as follows:

• Once published, interfaces are immutable; that is, they do not change. You can rely on an interface to provide a specific set of functions. Additional functionality is provided by additional interfaces.
• By convention, COM interface identifiers begin with a capital I and a symbolic name that defines the interface, such as IMalloc or IPersist.
• Interfaces are guaranteed to have a unique identification, called a Globally Unique Identifier (GUID), which is a 128-bit randomly generated number. Interface GUIDs are called Interface Identifiers (IIDs). This eliminates naming conflicts between different versions of a product or different products.
• Interfaces are language independent. You can use any language to implement a COM interface as long as the language supports a structure of pointers, and can call a function through a pointer either explicitly or implicitly.
• Interfaces are not objects themselves; they provide a way to access an object. Therefore, clients do not access data directly; clients access data through an interface pointer. Windows 2000 adds an additional layer of indirection known as an interceptor through which it provides COM+ features such as just-in-time activation and object pooling.
• Interfaces are always inherited from the fundamental interface, IUnknown.
• Interfaces can be redirected by COM through proxies to enable interface method calls to call between threads, processes, and networked machines, all without the client or server objects ever being aware of the redirection. For more information, see In-process.