A public key is one of the two keys used for asymmetric cryptography. In this manner of information encryption, there are two different keys that belong together: one for encryption and one for decrypting information. Unlike the secret key, the public key is meant to exchange with the one you want to communicate with.
An example of an algorithm that this uses is RSA. Examples of programs that may use this are SSH and Pretty Good Privacy applications.
Big advantage of two different keys is that one key can now be publicly disclosed, so everyone can use this key to edit the data. Often this is used to encrypt information, after which the now protected information can be sent to the owner of the key. How does it work?
Only this owner besides the public key is also his secret key that can only be used to convert encrypted information to readable text.
The core of the principle is that everyone can protect information with a public key in such a way that only the owner of the associated secret key can decipher the information. This guarantees the secret of the message. Only the recipient can decrypt the message with his secret key.
Conversely, the owner of a secret key can encrypt a message and send it to everyone. The recipients can now decipher the message with the public key and read the information. Because the information with only the public key was deciphering, the recipients are sure that the message from the secret key owner is coming from, and also that the message is not modified on the move. He is the only one who knows the secret key and can not deny being the source of the message. This is called non-repudiation in the literature.
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