Most computers have a floppy drive that can read floppy disks. Many PCs also include drives that can read and write much larger amounts of data from other types of removable disks, like CD burners or Zip disks.
Another type is the USB or Firewire (that are hot swappable) and can be used with external hard drives, tiny USB "keychain" drives, digital cameras and media card readers (i.e. flash memory).