Symmetric encryption uses the same key to encrypt and decrypt data. As an example, Remote Authentication Dial-In User Service (RADIUS) uses a shared key for symmetric encryption.
AES is a popular symmetric block encryption algorithm, and it uses 128, 192, or 256 bits for the key.
DES is an older, symmetric block encryption algorithm. 3DES was created as an improvement over DES and is used when hardware doesn’t support AES.
Diffie-Hellman provides a method to privately share a symmetric key between two parties. Elliptic Curve Diffie-Hellman Ephemeral (ECDHE) is a version of Diffie-Hellman that uses ECC to re-create keys for each session.