Everyone who once used an Apple II and now uses a PC has the same problem: How can you make the PC read Apple floppy disks? Unfortunately, without special hardware, you can't.
Floppy disks are analog devices, much like cassette tapes. For a computer to store digital data on a floppy disk, it must "encode" the data into an analog format. The Apple II used a method of encoding called Group Code Recording (GCR), while IBM-compatible PC's used the much more standard Modified Frequency Modulation (MFM) encoding. Since this is all done in hardware and cannot be bypassed, it is not possible for a PC program to "reprogram" the 5.25" floppy drive in such a way that it could read Apple-formatted floppy diskettes.
Therefore, instead of reading and writing disks directly, AppleWin uses disk images. A disk image is a single file, which you can store on your PC, which contains all of the data from an entire Apple diskette. AppleWin treats an image exactly as if it were a real floppy disk.
Included in the AppleWin distribution is the DOS 3.3 System Master disk dated "January 1, 1983" (DOS 3.3 System Master - 680-0210-A.dsk), a blank DOS 3.3 disk (BLANK.DSK) created using the same System Master disk and a modern ProDOS disk (ProDOS_2_4_3.po).
NB. ProDOS 2.4 is an updated version by John Brooks, including support for all Apple II's. More details can be found here ProDOS8.com.