  A causal-net is a finite acyclic directed graph. In this paper, we introduce
a category, denoted by $\mathbf{Cau}$ and called causal-net category, whose
objects are causal-nets and morphisms between two causal-nets are the functors
between their path categories. The category $\mathbf{Cau}$ is in fact the
Kleisli category of the "free category on a causal-net" monad. Firstly, we
motivate the study of $\mathbf{Cau}$ and illustrate its application in the
framework of causal-net condensation. We show that there are exactly six types
of indecomposable morphisms, which correspond to six conventions of graphical
calculi for monoidal categories. Secondly, we study several composition-closed
classes of morphisms in $\mathbf{Cau}$, which characterize interesting partial
orders among causal-nets, such as coarse-graining, merging, contraction,
immersion-minor, topological minor, etc., and prove several useful
decomposition theorems. Thirdly, we introduce a categorical framework for minor
theory and use it to study several types of generalized minors in
$\mathbf{Cau}$. In addition, we prove a fundamental theorem that any morphism
in $\mathbf{Cau}$ is a composition of the six types of indecomposable
morphisms, and show that the notions of coloring and exact minor can be
understood as special kinds of minimal-quotient and sub-quotient in
$\mathbf{Cau}$, respectively. Base on these results, we conclude that
$\mathbf{Cau}$ is a natural setting for studying causal-nets, and the theory of
$\mathbf{Cau}$ should shed new light on the category-theoretic understanding of
graph theory.
