A branch of mathematics divided into two general fields: differential calculus and integral calculus. Differential calculus can be used to find rates of change, like orbits of planets, satellites, and spacecraft. Integral calculus is a method of calculating quantities by splitting them up into a large number of small parts. It can be used to find the surface area of irregular objects. You can find out the total surface area of your car (even the round parts) by using integral calculus. Source: Children's Encyclopedia Britannica vol. 3, p. 308-309, 1989.
Calculus is basically a means of plotting a curve, or a slope (which is the same thing). Initially it was used to plot the orbits of planets but many phonomania or objects can be represented as curves, or collections of curves so it has wide application in science, economics, and design.