Higher rank graphs are objects extending the idea of a directed graph. Their inception more than two decades ago is directly traced back to certain constructions of C*-algebras from affine buildings. The definition of a higher rank graph is in categorical terms. As such, it is deceptively simple yet extremely powerful. C*-algebras associated to higher rank graphs have attracted much interest and have now a rich theory. Outside of operator algebras, higher rank graphs have been used, among others, towards new constructions in group theory. In this talk I will motivate the concept of a higher rank graph and some of its uses. Two lines of development will be highlighted, one being the study of KMS, or equilibrium, states, the other being higher rank graphs constructed from groups acting on products of trees and aspects of their spectral theory.