Although part of the same series of standards, SHA-3 (Keccak) is internally different from the MD5-like structure of SHA-1 and SHA-2. -- Keccak is based on a novel approach called sponge construction. Sponge construction is based on a wide random function or random permutation, and allows inputting (`absorbing` in sponge terminology) any amount of data, and outputting (`squeezing`) any amount of data, while acting as a pseudorandom function with regard to all previous inputs. This leads to great flexibility. (from wikipedia)