1) Substitution ciphers encrypt messages by replacing each letter in the plaintext with another letter according to a fixed mapping or key. The same key is required to decrypt the ciphertext back to the original plaintext. 2) Caesar ciphers are one of the earliest and simplest substitution ciphers, where each letter is shifted a fixed number of positions down the alphabet. While they allow secret communication, the encryption is weak and vulnerable to frequency analysis attacks. 3) Frequency analysis works by comparing the letter frequencies in the ciphertext to the expected frequencies in the language the plaintext was written in, like English. This reveals the most common letter mappings used in the simple substitution cipher key.