What is a different between digital signature and ElGamal ?

Answer 1

ElGamal is an asymmetrical (public key) cryptosystem - like RSA. Like all such systems, it can be used both for encryption and for digital signing.

The Digital Signature Algorithm (a NIST standard) is a slight variant of ElGamal (when used for signing) which uses SHA-1 as a hash function and slightly different formulae - see the two algorithms described on the Web pages pointed by the links below.