A substitution cipher is a very simple (and easily broken) cipher in which every letter of the alphabet is replaced by a different letter. For example, each letter might be replaced by the letter below it:
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z E F G H I J K L M N O P Q R S T U V W X Y Z A B C D
Then "CONSERVAPEDIA" would encrypt into "GSRWIVZETIHME."
In the above example, each letter is replaced with the letter that comes four places after it in the alphabet. This kind of cipher has the advantage that the encryption system can be memorized: all that needs to be remembered is that A encrypts into E and the rest follows automatically. However, this kind of substitution cipher (known as a Caesar cipher) is extremely weak. There are only twenty-five possible Caesar ciphers, so if the cryptanalyst knows or suspects that the message has been encrypted with a Caesar cipher, the message can be decrypted simply by trying each of the twenty-five possibilities.
The strongest possible substitution ciphers replace each letter with a random substitute. But even these are easily broken by simple analyses of letter and digram frequencies. In fact, newspaper "cryptograms" are nothing more than substitution ciphers, and readers are expected to solve them in an hour or so.
Each letter can be replaced by an arbitrary symbol rather than a letter, as in this example from the Sherlock Holmes story The Adventure of the Dancing Men:
Logically, this does not make the substitution cipher any harder to break, but it may make it look more puzzling, or even disguise the fact that a secret message has been encrypted.