What do the different types of compression do?
I don’t really know. LZW is a pretty efficient compression algorithm, but there are better ones available now. The FAX-CCITT compression is the same as used by fax machines, and only works on one-bit images (black and white with no gray). Your faxing program and OCR programs should be happiest with fax-compressed images, but most recent ones can handle many kinds of TIFF files. If you really need to know, look in the comp.compression FAQ at http://www.faqs.org/faqs/by-newsgroup/comp/comp.compression.html, or find a textbook on data compression.