Private Key:
In Private key, the same key (secret key) is used for encryption and 
decryption. In this key is symmetric because the only key is copy or 
share by another party to decrypt the cipher text. It is faster than 
the public key cryptography.

Public Key:
In Public key, two keys are used one key is used for encryption and another 
key is used for decryption. One key (public key) is used for encrypt the plain
text to convert it into cipher text and another key (private key) is used 
by receiver to decrypt the cipher text to read the message.