This article is from the Apple ][ Emulator Resources Guide, by Alex Maddison with numerous contributions by others.
Apple II 5.25" diskettes are not compatible with that 5.25" floppy in the front of your PC. Period.
A disk-image is a track-by-track copy of a physical diskette made to a fixed volume as a software file; basically there is little difference between it and a compressed archive (eg. a PKZIP or StuffIt file). In everyday use, this is useful because of a number of factors - an "image" of a diskette retains more information than an archived copy (eg. volume name, original file dates), physical diskettes may become defective, it makes duplication of master- diskettes easier, disk-images can be stored on hard-disks eliminating the need for unwieldy numbers of diskettes, etc. All of the modern computing platforms use disk-images for storing and transferring disks over electronic networks (such as the Internet). Apple distributes its system software in DiskCopy image format (which is compatible with programs such as ShrinkWrap and MountImage allowing the image to be accessed like a real disk from the Finder). The PC uses WinImage (which is compatible with DiskCopyFast in DOS). Finally, the Amiga uses DiskMasher images. None of these image types are compatible with each other.
Emulators use disk-images for a totally different purpose. Whereas modern computers use images to transfer physical diskettes electronically, emulators use disk-images because the original media is not compatible with the modern disk drives. Although the diskette media itself is the same, the method used to store data on it is totally different. Disk-images are used to trick the emulator software into thinking that there is an Apple ][ disk-drive attached (unlike modern software, most Apple ][ applications could not be installed onto a hard-drive and were dependent on the 140K diskette structure to operate). Unlike the different types of images above, emulator images can be used on any computer. There are six types of Apple ][ disk-images, of which three are standard to virtually every emulator on every platform... But why have more than one type of disk-image to emulate an Apple disk-drive? Because any disk-drive can access the information on a diskette in a number of ways. An Apple ][ drive alone supports DOS and ProDOS structuring, as well as a Nibble format which included half and quarter tracks used in some software as copy-protection. These are the three main types which are standard to most emulators.
For further information on disk-images, please refer to Chapter 6 of this guide.