Graph

A graph is a data structure $$K$$ consisting of nodes $$\mathcal X = X_1, \dots, X_n$$ and edges between the nodes, $$K=(\mathcal X, \mathcal E)$$. A pair of nodes $$X_i$$ and $$X_j$$ can be connected by a 'directed' edge $$\to$$ or an undirected edge $$\$$.

Types of Graphs
There are two often used classes of graphs, namely (i) 'directed acyclic graph' (DAG), which is also often called a 'directed Bayesian network' and (ii) 'undirected graph', which is also often called a 'Markov network.'