  In this paper the structure of the Drinfeld realization $\Udr_q$ of affine
quantum algebras (both untwisted and twisted) is described in details, and its
defining relations are studied and simplified. As an application, a
homomorphism $\psi$ from this realization to the Drinfeld and Jimbo
presentation $\U_q^{DJ}$ is provided, and proved to be surjective.
