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Cryptography is a fascinating field which is all about securing communication through the use of codes so that only the intended receiver can understand the message. It's like a secret language that protects information from being intercepted by unauthorized parties.

At its heart, cryptography involves converting plain text, which is readable by anyone, into ciphertext, a form that hides the true meaning. This process is called 'encryption'. Conversely, 'decryption' is the process of turning the ciphertext back into plain text, which the authorized recipient can understand.

Modern cryptography uses complex algorithms and mathematical theories to ensure security. These algorithms work as the rules that govern how the encryption and decryption processes happen. It is essential not only in fields like military and government communications but also in protecting our day-to-day digital transactions like emails, bank transactions, and personal messaging.

A 'secret code' is the basis of making a message secretive and is a fundamental aspect of cryptography. It's a set of rules or a system which dictates how to transform the contents of a message to conceal its meaning. Imagine you're writing a secret diary, and you don't want your siblings to read it. You could develop a secret code where each letter of the alphabet is represented by a different symbol or number.

Secret codes have been used throughout history, from ancient Egyptians with hieroglyphs to spies during wartime. In fact, the use of secret codes is not limited to high-stakes scenarios; it could be as simple as kids playing and creating their own code to communicate secretly. The key to any good secret code is that it should be difficult to crack if you don't have the key or the rule to decipher it.

Coded messages are the result of applying a secret code to normal language. In essence, these are messages that are no longer in their original, plain format; they have been transformed into ciphertext. For someone who stumbles upon a coded message without the key or knowledge of the encryption method, the message appears like gibberish or nonsensical symbols.

Coded messages can be found in many places, from the classic spy's notes to the modern digital world, where encryption safeguards our sensitive data. They can be as complex as encoded military communications, which may require machines and computers to decode, or as straightforward as a substitution cipher used in a school treasure hunt.

An important thing to remember about coded messages is that for effective communication to occur, the recipient must know how to decode or decrypt the message back into a format they can understand, which implies the existence of a shared secret or key between the sender and the recipient.