Conventional cryptography consists of a single mathematical key used for both encryption and decryption of data. If you want to send a secure message to an addressee, you encrypt the message using a key known only to the sender and the recipient and then you pass both the key and encrypted message to the recipient so that the message can only be decrypted by the intended recipient.

Public Key Cryptography uses two keys. One key is kept private and the other key is made public. If the Public Key is used to encrypt a message, the Private Key can decrypt the message. In other words, if you want to send an addressee a message, you encrypt the message with the addressee's Public Key and pass the message to the addressee. The addressee can then use the Private Key to decrypt it.