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.
The driver topic is a very advanced one.
To get an overview, you can have a closer look on the Windows Driver Kit (WDK) sides.
If you decide to get into this stuff, then you need a lot of time, frustration resistance and fanaticism.
The first thing you should do (befor you touch the WDK!) is, to start reading a good book.
If you want to develop for windows file system, read Rajeev Nagar's book "Windows NT File System Internals : A Developer's Guide". It's published in 1997, but it's something like the "bible" of NTFS.
For common driver developement you can find books like "Developing Drivers with the Windows Driver Foundation", written by Penny Orwick.
These books describes programming kernel mode software, which is done in C language. So, you should have a good base knowlege on C before you start.
Among others there are the OSR side (www.osr.com) and SysInternals on technet (http://technet.microsoft.com/de-DE/sysinternals), which are truely worth to have a closer look on.
More than the halfe time you spend on reading debug outputs and crash dumps, so it's wise to know what these things are meaning and how to get this information, but there are good books for windows debugging too.
I hope, I was able to give a short overview on the question for the ways to learn more.
来源：https://stackoverflow.com/questions/11438859/are-filter-drivers-intended-to-extend-system-drivers