Magnetic hard disks have been around since the 1950s and are now in computers, MP3 players and video cameras. Optical storage is somewhat newer, coming out of videodisks and the compact disk in the 1980s. Both technologies are still evolving rapidly, offering more storage space at lower costs.
Magnetic disk storage has been optimized for high speeds. DVDs and CDs access data fast enough to play movies and music, but are much slower than a magnetic disk.
Since the disk is small, light and removable, optical media is portable. Most magnetic disks, while lightweight, are meant to stay inside the computer.
Neither magnetic hard disks nor optical disk drives are expensive, though optical drives go for a fraction of what a hard disk costs.
As of 2009, magnetic disk drives with trillion-byte (terabyte) capacities had become common. Individually, dual-layer Blu-Ray optical disks store 50 billion bytes (gigabytes).
Magnetic disks have been a workhorse technology for computers since the 1950s. They're the main place to keep files and software. Optical disks are handy media for portable data, movies and music.