From Just Solve the File Format Problem
RC4 (also called ARC4 or ARCFour) is a cryptographic stream cipher, designed by Ron Rivest in 1987. It was originally a trade secret that was licensed for use in proprietary software. It was leaked or reverse-engineered in 1994.
RC4 is unusual in that it is purely a stream cipher, not a CBC (cipher block chaining) mode of a block cipher.
Though not completely broken (as far as we know), several cryptographic weaknesses have been found in it. Its use is discouraged.