Entropy Encoding - is a lossless data compression scheme that is independent of specific characteristics of the medium. One of the main types of entropy coding creates and assigns a unique prefix-free code to each unique symbol that occurs in the input. These entropy encoders then compress data by replacing each fixed-length prefix-free output codeword. ( Most common symbols use the shortest codes).
Arithmetic Coding (rather than separating the input into component symbols and replacing each with a code, arithmetic coding encodes the entire message into a single number, an arbitrary-precision fraction q, where 0.0 <= q < 1.0.)