Disk storage was, until recently, the primary form of high-speed on-line secondary storage for computers. ('Secondary' because, unlike main memory, the data on it was not directly accessible to the CPU; rather, it has to be read into memory before the CPU can look at it.)
All disk storage is basically the same; a circular 'platter' is coated with a magnetic coating - very fine particles of magnetic material mixed with a binder, which caused the coating to adhere to the platter. A read/write head, identical in basic concept to the head on a magnetic tape recorder, is used to read and write data on the disk as the disk is rotated.
A separate mechanism is used to move the head in and out; with the head at a fixed position, it writes a track. Many disks have more than one platter, each with its own read-write heads (usuall two, one for each surface), and the collection of tracks at a given offset is referred to as a cylinder. Each track is usually divided into a number of sectors - a complete track usually contains too much data to be usefully read and write as a complete unit.
Initially, all disks used metal platters (usually aluminium). IBM developed a new form of disk, the floppy disk, which used instead a platter of mylar (a stiff plastic). This was carried in a stiff plastic envelope with a felt layer on the inside. The first floppies were 8 inches in diameter; later they were produced in 5-1/4 inch, and finally 3-1/2 sizes (the latter in rigid plastic containers).