Microsoft® Windows® 2000 Scripting Guide
Binding refers to the way that a script or an application accesses a COM object. When you create an object reference to an Automation object, VBScript must verify that the object exists and that any methods or properties you attempt to access are valid and are called correctly. This process of connecting to and verifying an object and its methods and properties is known as binding.
COM supports two types of binding: early and late. With early binding, an object, its methods, and its properties are checked when the application is compiled. If there are any problems, compilation will fail. Early binding is faster than late binding because the object is verified before the application runs. In addition, early binding provides access to the objects type library, which contains information about the methods and properties of the object. The information in the type library can then be included within the compiled code and thus be available whenever the application needs it.
Because VBScript is not a compiled language, it does not support early binding. Instead, you must use late binding, in which binding does not occur until the script actually runs. With late binding, the script must access the registry to obtain information about the object, its methods, and its properties. Because VBScript does not have access to the objects type library, it must perform a similar lookup any time it accesses the object or attempts to use one of the objects methods or properties. In addition, any incorrect calls to the object will not be found until the script actually runs.