What's the difference between the Windows Management Instrumentation and the Registry?

"WMI isn't really anything to do with or related to the registry. It is simply a way to manipulate and manage Windows machines both locally and over the network - this includes registry tasks, but it also includes a lot more.

Here is the official guide about WMI from Microsoft.

Windows Management Instrumentation (WMI) is the Microsoft implementation of Web-based Enterprise Management (WBEM), which is an industry initiative to develop a standard technology for accessing management information in an enterprise environment. WMI uses the Common Information Model (CIM) industry standard to represent systems, applications, networks, devices, and other managed components. CIM is developed and maintained by the Distributed Management Task Force (DMTF)."