The Playfair Cipher is a polygraphic substitution cipher that encrypts pairs of letters at a time, instead of single letters. It uses a 5x5 grid of letters, where the letters of a keyword are used to fill the first row and first column of the grid, and the remaining letters of the alphabet are filled in the rest of the grid. To encrypt a message, each pair of letters is mapped to a different pair of letters on the grid, using a set of rules based on their position in the grid. The Playfair Cipher was invented in the 19th century and was used by the British army during World War I and II.