The Vigenère cipher is a method of encrypting alphabetic text by using a simple form of polyalphabetic substitution. It employs a keyword, where each letter of the keyword corresponds to a shift in the alphabet for the letters of the plaintext. For example, if the keyword is "KEY" and the plaintext is "HELLO," the letters of the keyword are repeated to match the length of the plaintext, allowing for a unique shift for each letter. This cipher was developed in the 16th century and is named after the French cryptographer Blaise de Vigenère. Although it was considered secure for many years, it can be broken using frequency analysis techniques, especially when the keyword is short or repeated. Today, it serves as an educational tool for understanding basic encryption concepts.