A standard Fork Attack sees one Pawn or Piece attack TWO enemy units, in a single move – hence the alternative term “Double Attack”, which some use when referring to Fork Attacks.
The following image shows an example of a standard “Rook Fork”, by White, against Black’s Knight and Pawn:
By comparison, the Grand Fork attacks the enemy King, Queen and either one or both Rooks – at the SAME time, from ONE move. This can be seen in the following image::
With its unique L-shaped range of movement, the Knight is usually the best of all Pieces for carrying out a Grand Fork Attack, as it’s usually “just out of reach” from becoming attacked by one of its 3 or 4 victims.