An HTML document that has multiple components that interact with one another is said to have active content. In a way, active content is an extension of object-oriented programming: the components communicate with one another by way of public interfaces. You can think of HTML as the binder that holds all these components together, making them aware of one another and passing information back and forth among them. Such interoperability requires the component interfaces to conform to a widely accepted standard. The Component Object Model (COM) is the standard that was developed for this purpose. COM is described in detail later in this chapter and even more thoroughly in Chapter 6, but first some background discussion is in order.