Q: Terminology : encoding without decoding? I am having trouble understanding such basic concept.
I did some research about cryptography and manipulated few concepts (RSA key pair, AES/DES/whatever secret key, hash functions ...). But I would like to understand more deeply one basic thing :
Encoding is transforming a message into an other form.
Decoding is giving a message its original form.
Well, for me it sounds like encryption is encoding. And I think (please correct me) that encrytion is a way of encoding (for a very particular purpose : increasing the confidence of having a known list of person who can decode).
But what about hash function ? Since there is no decoding function, when we hash a message, can we say :
"this text is this message encoded with SHA-1 algorithm", 
as we can surely say :
"this digest is this message hashed with SHA-1 algorithm"  ?
Thank you !

A: Encoding, and its reverse decoding, are mere transformations of the data into some alternative form. Each form expresses the exact same data, just written differently. The transformation is well known and can be carried out by anyone.
Encryption, and its reverse decryption, is the encoding of data using a secret. The ciphertext (the encrypted data), is for all intents and purposes random noise. The ciphertext does not express the plaintext in some alternative format, the plaintext is hidden inside the ciphertext. The transformation is not well known, as it requires a secret key which, supposedly, only specific entities are in possession of.
In that way, yes, encryption is a specialised form of encoding, but in usage "encoding" typically means a transformation that can be carried out by anyone, while "encryption" specifically involves preventing unauthorised parties from carrying out the transformation.
Hashing is a one-way operation (there's no dehashing) and thereby entirely distinct from the other two operations.
