This article is from the Miscellaneous Macintosh FAQ, by Elliotte Rusty Harold email@example.com with numerous contributions by others.
This isn't as frequently asked a question as it used to be since
Apple started bundling Macintosh PC Exchange with System 7.5.
As long as Macintosh PC Exchange is loaded any Mac with a
Superdrive (that is all Macs sold since the introduction of
the IIx in 1990) can read, write and format 3.5 inch PC floppies.
Macintosh PC Exchange does not support Windows 95's long file names
though. For that you'll need the commericial product Dayna DOS
System software versions 6.0 though 7.1 include Apple File Exchange
instead, a minimal program to read, write and format 3.5 inch PC
floppies in a Superdrive. Apple File Exchange is difficult to use
and violates at least half of Apple's user interface guidelines.
(Can anyone explain why no other software company violates as
many of Apple's user interface guidelines as Apple itself does?)
If you don't have a Superdrive, the easiest way is to transfer the
files across the Internet or a LAN. If that's not an option,
perhaps because you'rue transferring files from a really old DOS box
and you don't want to waste your time trying to get it to talk to
your ISP or network, then you can always move the files between two
computers with a null-modem cable connected between their serial
ports and a reliable communications program. You can get a
null-modem cable from any good electronics store. Make sure the
cable you buy has the appropriate connectors for the Mac and PC
you'll be connecting. Hook one end of the cable to the printer or
modem port on your Mac and the other to a serial port on the PC.
This should work just like a very high speed (57,600 bps) modem
connection except that you'll probably need to turn on local echo in
your communicatins programs.