Magnetic disks tend to come in two varieties: the hard metal platters inside hard drives and the more flexible magnetic platters inside floppy disks. As of 2014, most hard drives use either 3.5- or 2.5-inch platters, while the only floppies still in use are 3.5-inch designs.
Hard drives designed for use in desktop PCs tend to use the 3.5-inch form factor, while the smaller 2.5-inch drives are designed for use in laptops. Some specialty desktop designs, especially mini ITX systems and small machines, may also use the 2.5-inch form factor to save space.
While 3.5-inch floppy disks are still manufactured as of 2014, most computers do not ship with a floppy drive installed, and many motherboards do not even support the technology anymore.