Alternatively referred to as a diskette, a disk is a hard or floppy round, flat, and magnetic platter capable of having information read from and written to it. The most commonly found disks with a computer are the hard disks and floppy disks (floppy diskette) shown in the picture to the right.
When using the term "disk" or "diskette," you're referring to a floppy diskette or hard disk drive. However, when talking about an optical disc (e.g., Blu-ray, CD, or DVD) use "disc" and not "disk." For example, saying "my movie is on that DVD disc" is the proper usage of "disc." An easy way to remember this is that a "disk" is always a magnetic storage using magnets, and a "disc" is always an optical media using a lasers or lights.