MO discs, or Magneto-Optical discs, are a type of storage medium that combines magnetic and optical technologies. They use a laser to heat a small area of the disc, allowing a magnetic field to change the polarity of that area, which encodes data. This process makes MO discs rewritable, meaning data can be erased and rewritten multiple times. These discs typically come in 3.5-inch and 5.25-inch formats and can store various amounts of data, ranging from 128 MB to several gigabytes. MO discs were popular in the 1990s and early 2000s for data backup and transfer, but have since been largely replaced by USB flash drives and cloud storage.