Calculus is the study of rates of change. There are two branches of calculus: differential calculus, which determines the rate of change of a quantity, and integral calculus, which finds the quantity where the rate of change is known. Calculus is useful for finding slopes of curves, calculating areas of irregular shapes, justifying formulas, finding complicated roots, visualizing graphs, finding average values of functions, and calculating optimal values. It allows us to determine slopes of curves, calculate areas of bizarre shapes, justify old geometry formulas through calculus processes, find irrational roots through methods like Newton's method, visualize graphs of functions, and calculate average heights, velocities, and accelerations of functions.