DSK (Apple II)
The Apple II DSK format (not to be confused with the Amstrad DSK (CPCEMU) format) is a group of formats reproducing the sector structure of floppy disks for the Apple II series, including Apple II 16 sector disks and Apple ProDOS disks, usually containing files in an Apple DOS file system or ProDOS file system.
The .dsk extension can technically refer to any of several formats in this family, but in practice it is predominantly used for 5 1/4" disk images in DOS 3.3 sector order, and is the most common extension for such disk images.
The .do extension refers to 5 1/4" disks in DOS 3.3 sector order, but this extension is seldom encountered since .dsk is usually used instead.
The .po extension refers to disk images in ProDOS sector order.
The .hdv extension refers to larger disk images, including images of 3 1/2" disks and hard disks, in ProDOS order.
The "DOS 3.3" vs. "ProDOS" orders refer to the order of the sectors in the file corresponding to how those respective operating systems arrange sectors on the disk, but the actual data might not be for the same operating system as their sector order implies.
A normal 5 1/4" disk image contains 35 tracks, each with 16 sectors of 256 bytes each, for a total of 143,360 bytes. The bytes are stored in these files in their raw form, with no headers or separators added.