A letter or number are represented by different symbols in the mesage. The most famous of these ciphers is the Vigenere cipher. With the Vigenere cipher you have a key word (ie., BAD). Each letter in the keyword refers to a different ordering of the alphabet, ie.,

B C D E F .............................................. Z A

A B C D E ...............................................Y Z
D E F G H .....................................Z A B C

The first letter in the message comes from the B alphabet, the second from the A alphabet and the 3rd from the D alphabet. This process is then cycled through again for the 4th, 5th and 6th letters and so on till the end of the mesage.

Lets say the message starts, The bus and the coat are in the box'.

The cipher will read

U H H C U V B N A U H H D O D U A U F I Q U H H C O A.

Note that 'U H H' is 'the', which is repeated three times because each 'the' starts with the first letter in the cycle (ie., B in BAD). If it had started on the 2nd or 3rd letters then THE would have been

T K F or W I E

This cipher was invented by several people 15th and 16th Centuries. Vigenere was a French Diplomat who retired early to be an intellectual. The first person to break this cipher that we know of was Charles Babbage.