XChaCha20 is a stream cipher based on ChaCha20 but with an extended nonce. ChaCha20 is based on the Salsa20 cipher (see below) but has improved security and performance.
XChaCha20 uses 20 rounds and a 192-bit nonce, whereas ordinary ChaCha20 uses a 64-bit nonce. ChaCha20 is immune to timing attacks.
XSalsa20 is a stream cipher based on Salsa20 but with an extended nonce. Salsa20 uses 20 rounds and a 64-bit nonce, whereas XSalsa20 uses a 192-bit nonce. Salsa20 is immune to timing attacks.
Curve25519 is one of the fastest elliptic curves and provides 128-bit security. Curve25519 is used for asymmetric encryption in varaious end-to-end encrypted messaging applications such as Signal as well as network protocols like TLS. Curve25519 is immune to timing attacks.
XSalsa20-Poly1305 is the authenticated version of XSalsa20 that uses Poly1305 as a MAC to verify the authenticity of a message.
Click here to read more about the libsodium Sealed Boxes construction.