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Most ciphers — both classical and modern — will work just fine with any key. It's just that, if the key used to dechipher the message does not match1 the key used to encipher it, the output will be essentially nonsense, and the actual intended message will not be revealed. (Some encryption systems may then detect that the decrypted text is ...
I personally have always seen a key used for encryption as a key used in a door, I never compared it to a keystone. But i think it is fair to compare it to a key with which you open a door. Since a key in cryptographic sense give you access to data or even to complete systems. Further more a keystone in the sense that a key is needed to make it work is not ...
fkraiem's definition is too narrow. $\:$ "In the context of encryption schemes," keys are "whatever piece of information the legitimate recipient of an encrypted message possesses, which allows him to decrypt the ciphertext" and any information related to keys of the type mentioned above, which allows its possessor to encrypt the plaintext .
In the context of encryption schemes, the key is whatever piece of information the legitimate recipient of an encrypted message possesses, which allows him to decrypt the ciphertext efficiently. Hence, the key must be kept hidden from an attacker, since otherwise the attacker could decrypt efficiently just as the legitimate recipent does.
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