The Caesar Cipher is a symmetric (Private Key) crypto system first used in the ancient Roman Empire to encrypt sensitive messages. It is a substitution cipher, this means that we shift all the letters of the plaintext with a fixed number of letters. The private key is the value of which we shift the letters, and so, this is not a very strong cipher and is rather easy to work out and decrypt.
We can easily create a Caesar Cipher by taking the letters of the alphabet and assigning each of them an integer value. This allows us to then shift the letters by X units to encrypt our data. When we decrypt we do the opposite, by deducting the private key from the letters assigned integer.