Book excerpt: Boundary value problems are differential equations with a set of additional constraints, called the boundary conditions. A solution which satisfies the boundary conditions is a solution to the boundary value problem and the differential equation. The Weyl distance function behaves in some ways like the distance function of a metric space. It takes values in a group of reflections, instead of taking values in the positive real numbers. This group of reflections is called the Weyl group. A differential operator is a function of the differentiation operator, also known as the derivative. Differentiation can be considered as an abstract operation that accepts a function and returns another function. This book provides significant information of this discipline to help develop a good understanding of boundary value problems, Weyl functions, differential operators and related fields. It presents this complex subject in the most comprehensible and easy to understand language. Constant effort has been made in this book, using case studies and examples, to make the understanding of these difficult concepts of mathematics as easy and informative as possible to the readers.