What is a public key?
A public key is the key used to encrypt a message in asymmetric encryption. Unlike the private key, the public key can safely be shared with anyone without compromising the security of the message.
How does a public key work?
Public keys encrypt messages and files in one direction, meaning the public key used to scramble a file or message cannot be used to unscramble it. Decryption can only be accomplished by the receiver who holds the private key.
Public keys are often stored on local machines and on publicly accessible key servers. MIT, for example, hosts a searchable PGP key server used to encrypt email.