The Caesar cipher is the simplest subsitution cipher. It consists of applying a constant position shift to all the letters of a given plaintext. I also linked the ROT13 cipher to this article, since it uses the exact same logic. The letters A,B,C with a caesar shift (ROT) of 3 will respectively return the ciphered letters D, E, F. The ROT13 and ROT ciphers use the same logic.
Decoding the Caesar cipher can be easily achieved by using Brute Force guessing (where the ciphered text is passed through every value of the shift), or by frequency analysis.