A 'differential equation' is just an equation that has a derivative in it. 'just an equation' means these equations are simple equations to describe but solving them can be extremely challenging. There are whole college classes dedicated to studying just one type of these equations. We're going to wrap up our discussion with one of the simpler forms, called a 'separable' equation, that can be solved using the methods we've been discussing in this chapter and that will give you an idea of what differential equations are and how they can be used in practice.