This document introduces differential equations, including definitions of ordinary and partial differential equations. Ordinary differential equations relate a function to one independent variable and its derivatives, while partial differential equations relate a function of two or more variables to its partial derivatives. The document discusses the order and degree of differential equations, and explains that the order is the highest derivative and the degree is the highest order of the derivative. It also defines the solution to a differential equation, initial value problems, and the difference between general and particular solutions.