Salsa20 is a new variable-round stream cipher developed by Dan Bernstein in 2005. It is used in many Internet applications as a replacement for RC4 that has been shown to have statistical defects. ChaCha20 is a modified version of Salsa20 specified in RFC 7539 together with the Poly-1305 message authentication mechanism. As such, it is one of the five standardized AEAD ciphers that can be used in TLS 1.3. You may download a 10 page description of Salsa20 and ChaCha20 here.