The cairo DLL in cairo-1.2.6-win32-only.zip is built with only the
win32 surface and font backends. It has no external non-system
dependencies.

The cairo DLL in cairo-1.2.6.zip includes also the svg, pdf, ps and
png surface backends. It thus requires the libpng and zlib
distributions from the dependencies subfolder.

GTK+ 2.10 requires cairo with the pdf backend (because of the printing
functionality). GTK+ 2.8 doesn't.

The cairo 1.2.6 developer zipfiles are otherwise identical, but
obviously the cairo-features.h file (which defines macros for each
included backend) is different.