[Zope] Pluggable Brains?

People, Can someone point me to some documentation, messages or source code that will give me a handle on what 'pluggable brains' are and what I can do with them? Rob Page has just made a post saying that Zope/ZODB3/pluggable brains can go a long way to integrating with relational databases and I want to make sure I've got a clue about all this! :) thanks tone out,