A simple substitution cipher, also called monoalphabetic substitution cipher, replaces single letters separately with the ones specified in a ciphertext alphabet, also called substitution alphabet, which is fixed over the entire message. The combination of the plaintext and ciphertext alphabet forms the key of this cipher.
Caesar cipher rotates the alphabet by a specified number of places. It is one of the simplest and most widely known encryption techniques.
ROT13 rotates the Latin alphabet by 13 places and is thus a special case of Caesar cipher. With 26 letters in the basic Latin alphabet, it is its own inverse. Common variants of this cipher include ROT5, ROT18, ROT47, and ROT8000.
Atbash takes the plaintext alphabet and maps it to its reverse. It was originally used to encrypt the Hebrew alphabet.
Affine cipher uses a linear mathematical function to map a plaintext alphabet to a ciphertext alphabet.
Although the number of possible substitution alphabets is very large, this cipher can easily be broken in a ciphertext-only scenario. This is done by counting how often each letter appears in the ciphertext and using the resulting frequency distribution to guess its meaning.
This procedure is called frequency analysis and is based on the property that some letters or combinations of letters occur more often than others in natural languages.