Cryptology is the art and the science of developing methods to encode information and keep it secret from third parties.
There are two basic encryption methods: substitution (mono- and polyalphabetic) and transposition. The characteristic of substitution is that characters or groups of characters are replaced by other characters or symbols. For transposition the characters get reordered.

Already in the third millennium BC, the Egyptians used cryptography. The priests saw themselves as an elite class who had to protect secret knowledge and keep it hidden from the public. So they used other hieroglyphics to encrypt magical texts about the Gods.

Also the Templar and the Freemasons used that method.

Templar

Freemason

With the Skytale the Greeks developed another encryption device. To decrypt the message you needed two rods of a certain diameter, one for the sender and one for the recipient. The encrypted message was written on a leather strip which was wrapped around the rod to make it readable. This method is still used in an altered way. To encrypt it, you choose a number as key.

Example: key = 4
Message: Cryptography is so easy
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/skytalemeth.png

Encrypted text: CTAIEROPSAYGHSSPRYOY

The Caesar Cipher is named after Julius Caesar who is said to have used this encryption to communicate with his generals. A simple alphabet rotation is used. To every letter a number is mapped and encrypted by addition of the key.
Example: start C, key = 2,
Message: secret message
Method: E(x)=a+x mod 26

halls-of-valhalla.org/images/articles/CPUKillerEncryption/caesarmeth.png

Encrypted text: ugetgv oguucig

The Multiplication Chipher uses multipliaction instead addition as the Caesar Cipher does.
Example: key = 2
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/multimeth.png

Encrypted text: kieuim yikkami

Another variant of the Ceasar Cipher is the Affine Cipher.
Here the encryption is not only an addition but can be any mathematical function.
Example: key = 2x+1, A=10
Message: affine
Method: E(x)=f(x) mod 26

halls-of-valhalla.org/images/articles/CPUKillerEncryption/affinemeth.png

Encrypted text: lvvblt

Another old method is the Fence Cipher, which is another transposition cipher, and it has its name from the way it is encoded by writing it diagonally like an imaginary rail fence.
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/rails-fence.png

Encrypted text: SESEERTESGCMA

The Polybios Cipher is named by the philosopher historian Polybios who lived in the second century BC in Greece. Every letter in the square is represented by one number.
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/polybios.png

Encrypted text: key i=j 43 15 13 42 15 44 32 15 43 43 11 22 15

Many classical ciphers reveal statistical information about the secret message and that makes it possible to crack it. In any language each letter has a statistical frequency of usage.

To prevent this, they combined different methods or used keys.

Leon Battista Alberti, the inventor of the Alberti Cipher, was an Italian illegitimate son of a nobleman, who was not only famous of being a sophisticated scholar but also for his work as an architect.
Letters which are next to each other are encrypted in the Alberti Cipher with different keys.
Example: key1 = +2
key2 = *2
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/alberti.png

Encrypted text: uieugm oiukcmg

The Trithemius Cipher was invented by the monk Johannes Trithemius. Trithemius is the author of several books about cryptography and stenography.
In t[]his case the key is a word and for encryption a tabula is used.

Example: key = Valhalla
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/tablemeth.png

Encrypted message: nenyee xenslne

You also use a keyword for the Vigenère-Cipher, which was described for the first time in 1553 by Giovan Battista Bellaso in his book La cifra del. Sig. Giovan Battista Bellaso, but falsely credited to Blaise de Vigenère, who lived in the 19th century.

Example: key = Valhalla
Message: secret message
Method:

halls-of-valhalla.org/images/articles/CPUKillerEncryption/vigenere.png

Encrypted message: lalaah aallvla

A very well known encryption is Morse Code. It encrypts a message by using a combination of dots and lines. It can also be acoustically encrypted by using a series of on-off tones, lights, or clicks. It was invented by the American artist Samuel Morse, the American physicist Joseph Henry and Alfed Vail, 1836.
Message: secret message

Encrypted text: ... . -.-. .-. . - -- . ... ... .- --. .