  In this paper we study general quantum affinizations $\U_q(\hat{\Glie})$ of
symmetrizable quantum Kac-Moody algebras and we develop their representation
theory. We prove a triangular decomposition and we give a classication of (type
1) highest weight simple integrable representations analog to
Drinfel'd-Chari-Pressley one. A generalization of the q-characters morphism,
introduced by Frenkel-Reshetikhin for quantum affine algebras, appears to be a
powerful tool for this investigation. For a large class of quantum
affinizations (including quantum affine algebras and quantum toroidal
algebras), the combinatorics of q-characters give a ring structure * on the
Grothendieck group $\text{Rep}(\U_q(\hat{\Glie}))$ of the integrable
representations that we classified. We propose a new construction of tensor
products in a larger category by using the Drinfel'd new coproduct (it can not
directly be used for $\text{Rep}(\U_q(\hat{\Glie}))$ because it involves
infinite sums). In particular we prove that * is a fusion product (a product of
representations is a representation).
