[aelefson]

Hi-
While I do not collect T206s myself, here are some brief answers to your questions. I am sure others will provide more detail, but here is a quick response:
Piedmont 150 or 350 etc, are the number of images that were supposed to be in that series (it is the number printed on the back of the card such as 150 Subjects on the back of a Piedmont 150 series card in the top right corner). Scott Reader has an excellent book that can tell you a lot more about these series. They correspond to different printing dates within the 1909-1912 time frame.
Factory 25, 30, etc, are the factory numbers printed on the bottom of back of the card that shows which factory it was printed at. Some factories (like Piedmont 42) are harder to find than others).
Overprints (or overstrikes) are when the factory number is overprinted by an ornate red bar (normally) and a new factory number is added.
This is a very quick response. I do recommend you try searching this forum as their is a great deal of information on here about T206s. I also recommend checking the Archive section up top as well.
Good Luck,
Alan