What is automatic compression for PDF Formats

I have several questions relating to automatic compression, which come to the fore as I’ve tried to efficiently print out documents, and/or pre-existing PDFs, into smaller files.

I have 1.6.2, and note that the compression setting is "automatic"

1.   What is that, exactly?   The user manual doesn’t provide a clue
2.   How does compression of the Text or JPEG, inside a PDF, relate to DPI?   Why isn’t there a DPI setting?
3.   When I have tried to print out large-file-size PDFs, using PDFCreator, the printed file is no smaller than the original.   Does this have to do with the AutoCompression setting?   I try to view and then print out files which are scanned and emailed to me, and have file sizes which approximate 1-1.5 Mb per page!   All in an attempt to reduce that to less than 100k/page.
4.   Why is DPI resolution only available for setting when the compression is not automatic, -and- there -is- resampling
5.   What is the difference between downsampling and “average downsample”

6.  What is “Zip”, in the compression area ?