What is the difference between symmetric and asymmetric encryption?

Symmetric encryption uses the same key for both encryption and decryption. Fast and efficient for large data. The key must be shared securely — the "key distribution problem." Algorithms: AES (Advanced Encryption Standard — the standard, 128/256-bit keys), 3DES, ChaCha20. Asymmetric encryption uses a key pair: a public key (shared freely — anyone can encrypt with it) and a private key (kept secret — only the owner can decrypt). Much slower than symmetric — used for key exchange and digital signatures, not bulk data. Algorithms: RSA, ECC (Elliptic Curve), Diffie-Hellman. In practice, TLS combines both: asymmetric crypto is used in the handshake to securely exchange a symmetric session key; then symmetric encryption is used for the actual data transfer (fast + secure).