A disk is a thin, circular object used to store computer data (well, the actual disk itself is round, but it is stored in either a square, paper envelope or a square, hard plastic case). The disk goes into a disk drive which spins the disk very rapidly, allowing information to be located and transferred to or from the disk quickly.
The surfaces of floppy disks and hard disks are coated with a magnetic material similar to recording tape. Because hard and floppy disks store information magnetically, any magnet can destroy the data (information) on the disk. This means you must not get your floppy disks near a magnetic paper clip holder, the telephone, the stereo, a portable radio, or any other electronic device-and don't pin them to the filing cabinet with a magnet!
A double-sided disk is a floppy disk on which can store information on both sides. Double-sided is not the same as "double density"; some single-sided disks are double density on their one side. Neither "double-sided" nor "double density" are the same as "high density."