  In this paper, we decompose the set of fully commutative elements into
natural subsets when the Coxeter group is of type $D_n$, and study the
combinatorics of these subsets, revealing hidden structures. (We do not
consider type $A_n$ first, since a similar decomposition for type $A_n$ is
trivial.) As an application, we classify and enumerate the homogeneous
representations of the Khovanov-Lauda-Rouquier algebras of type $D_n$.
