Doron Holan’s blog has a guest post by Jake Oshins on IRQL that provids a nice summary on the concept:
http://blogs.msdn.com/doronh/archive/2010/02/02/what-is-irql.aspx
For those who aren’t aware, Jake has done lots of development work on the HAL and ACPI (amongst other things) so he’s the one that you want to talk to when it comes to core Windows concepts such as IRQL, interrupt handling, power management, etc.
[...] request levels are a fundamental Windows concept. We all know what they are (and if not we should)