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 talking about a disk or diskette, you're referring to a floppy diskette or hard disk drive. However, when talking about a Blu-ray, CD, or DVD you should 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.