[Zope] Pluggable brains?

Could somebody explain the concept of pluggable brains? I have come across this phrase more than once, and I still don't know what it is. Is there any documentation on this? Is it by any chance related to an SQL Method's ability to represent data records with Python classes? -- Alexander Staubo http://www.mop.no/~alex/ "Reality is that which, when you stop believing in it, doesn't go away." --Philip K. Dick