Flying

Flying is a evasion ability that makes creatures without flying unable to block creatures with flying. It has been in Magic since the original Alpha set. Flying is scattered wildly on the color wheel, also because it is a very popular keyword printed on over 900 cards. It is, however, mostly used on blue cards.

From the Comprehensive Rules

• 502.4. Flying
  • 502.4a Flying is an evasion ability.
  • 502.4b A creature with flying can’t be blocked by creatures without flying. A creature with flying can block a creature with or without flying. (See rule 309, "Declare Blockers Step.")
  • 502.4c Multiple instances of flying on the same creature are redundant.