What is a "rich-root"?

[Matt Doran]

Hi there,

I've been playing with bzr over the past few months, evaluating it as a 
replacement for svn. 

I see a lot of talk about rich-roots and I know it is required for 
bzr-svn.   Does anyone have a layman's description of what they are, and 
how they differ from standard formats?    What can it store that 
non-rich-roots can't?

It's probably not anything that I need to understand as a user ... but 
I'm curious. ;)

Regards,
Matt