Improper configuration of a device or software item can lead to system instability, data loss, or even complete system failure. Don't blindly waltz into your registry and start tinkering with settings. The registry contains configuration data for just about everything. Later upon boot, the system configures the device and inserts that data into the registry. The registry hands over the information that the driver wants, and everyone goes back to work.
Applications use the registry in a similar manner.
Suppose Microsoft Outlook needs to know where to look for your personal folder file. It consults the registry, which contains the path to the current user's file, and then opens up the file for use. The registry also contains the information that tells Windows what to do with a particular type of file, such as jpegs. The reference to classes implies that the software developers implemented an object-oriented approach to organizing the system data. Network configuration is also stored in the registry, but most of that could be grouped with the operating system information.