[MlMt] Junk vs. Spam: What's the difference? Why do I have both?

[Gary Hull]

Poking around in Sources I noticed that, in addition to Junk, some of my 
accounts have Spam folders. All the messages in them date to the last 
couple of months.

Is this something that the ISP is adding? Does it relate to my use of 
SpamSieve?

Since I do check the Junk folder for false positives, which do occur 
(just discovered that all mail from my sister was being discarded), 
should I also be checking the Spam folders, or modify the Junk 
conditions to include stuff in Spam?