A domination coloring of a graph $G$ is a proper vertex coloring of $G$ such that each vertex of $G$ dominates at least one color class, and each color class is dominated by at least one vertex. The minimum number of colors among all domination colorings is called the domination chromatic number, denoted by $χ_{dd}(G)$. In this paper, we study the complexity of this problem by proving its NP-Completeness for arbitrary graphs, and give general bounds and characterizations on several classes of graphs. We also show the relation between dominator chromatic number $χ_{d}(G)$, dominated chromatic number $χ_{dom}(G)$, chromatic number $χ(G)$, and domination number $γ(G)$. We present several results on graphs with $χ_{dd}(G)=χ(G)$.