A cipher is a form of message code, where letters or words are substituted according to a secret pattern. Knowledge of the pattern is needed in order to decrypt the message. Historically, a cipher might substitute certain letters for others, such as all Es for Gs, all Rs for Ys, etc, or replace letters with numbers or invented symbols, or rearrange the letters in words. A slightly more complicated cipher uses several letters to replace one, so E might be replaced with L, H or T. All these ciphers are relatively easy to crack.
During the Renaissance, the science of cryptology was studied more seriously, and more sophisticated ciphers were introduced. These involved a key (a variable combined in some way with the text) and an algorithm (a formula for combining the key with the text). A block cipher breaks a message up into several sections and combines a key with each section. These forms of cipher are much harder to break.