Non-linear functions are mathematical expressions where the relationship between variables is not a straight line when graphed. Unlike linear functions, which have a constant rate of change, non-linear functions can curve, bend, or change direction. Common examples include quadratic functions, which form parabolas, and exponential functions, which grow rapidly. These functions can model complex real-world situations, such as population growth or the trajectory of a thrown object. Understanding non-linear functions is essential in fields like physics, economics, and engineering, where they help describe behaviors that are not constant or predictable.