ChaCha is a family of stream ciphers proposed by Daniel J. Bernstein, as an evolution of Salsa20 with (conjecturally) improved resistance to cryptanalysis.
ChaCha, like Salsa20, is a family of stream ciphers, with the stream generator a hash function in counter mode. It uses 32-bit Addition, Rotation, and XOR (ARX), making it fast, constant-time (thus immune to timing attack), and lean on memory (thus cache-friendly when key changes often), on virtually any 32-bit (or wider) CPU.
ChaCha differs from Salsa20 by making twice as many updates to its state during its core function, and a few other changes, leading to faster diffusion. Also, the mapping of constants and counter in the input of the core function is revised.
ChaCha8, ChaCha12 and ChaCha20 are the variants intended to replace Salsa20/8, Salsa20/12 and Salsa20/20, respectively. As seen, the naming scheme has also been changed.