What are the differences between symmetric encryption and asymmetric encryption, and when is each used in network security?

Symmetric encryption uses a single shared key for both encryption and decryption, offering high performance but requiring secure key distribution. Asymmetric encryption uses separate public and private keys for encryption and decryption, providing better security but lower performance. Symmetric encryption is typically used for bulk data encryption, while asymmetric encryption is used for key exchange, digital signatures, and secure communication initiation.