Are filter drivers intended to extend system drivers?
Is this their main purpose?
Are they basically just an extra layer that sits between the driver and the user?
This seems overly simple an explanation and I am wondering if I am missing something.
Are there good ways to learn more?
In a way yes.
For example, if file system filter driver is for file encryption/compression/security, it is enhancing the file system functionality.
The filter driver does not handle talking to actual devices. They rely on lower level drivers to communicate with device. The filter drivers are add-on to the drivers to implement certain functionality. The active drivers which modify data/request are to enhance vanilla drivers while the passive filter drivers are just pass-throughs without any direct enhancements.
So I think your assumptions are correct. Will like to hear different views though.