Named after Julius Caesar— the Roman General who commonly used the cipher—The Caesar cipher is one of the best known (and most easily broken) substitution ciphers. It is known as a substitution cipher because letters are substituted for one another (commonly referred to as an offset or shift). In the example here, A is substituted for D, B for E, and so forth; the shift is 3. The key in this case is the size of the alphabet. It’s easy to break this cipher by trying all the substitutions.
(Check Out This IT Video: Build A Secure Future In Information Security)