A public key is a cryptographic key that can be shared openly and is used in asymmetric encryption. It allows anyone to encrypt messages or data that can only be decrypted by the corresponding private key, which is kept secret by the owner. This system ensures secure communication over the internet, as only the intended recipient can read the encrypted information. In the context of digital signatures, a public key also verifies the authenticity of a message. When someone signs a document with their private key, anyone with access to the public key can confirm that the signature is valid and that the message has not been altered. This process is essential for secure online transactions and communications.