The study of partial differential equations has been the object of much investigation and seen a great many advances recently. This is primarily due to the fact that certain classes of these equations fall under the category of being integrable. These kinds of equations have many useful properties such as the existence of Lax pairs, Backlund transformations, explicit solutions and the existence of a correspondence with geometric manifolds. There have also been many applications of solutions to these equations in the study of solitons and other objects which have seen applications in physics. It is the objective here to study some of these equations in a general way by using various ideas that have evolved in the evolution of the subject of differential geometry. The first sections give some introductory material related to the subject, and then the latter sections seek to apply these ideas to obtain many useful results with regard to nonlinear equations and to some examples of nonlinear equations in particular. Each chapter is self-contained and can be read on its own if desired.