[cryptking]

I see lot of answers indicating the meaning of DAG (Directed Acyclic Graph) but no answers on its applications. Here is a very simple one -

Pre-requisite graph - During an engineering course every student faces a task of choosing subjects that follows requirements such as pre-requisites. Now its clear that you cannot take a class on Artificial Intelligence[B] without a pre requisite course on Algorithms[A]. Hence B depends on A or in better terms A has an edge directed to B. So in order to reach Node B you have to visit Node A. It will soon be clear that after adding all the subjects with its pre-requisites into a graph, it will turn out to be a Directed Acyclic Graph.