Calculus (literally 'small pebble'), is the mathematical study of continuous change, in the same way that geometry is the study of shape and algebra is the study of generalizations of arithmetic operations. It includes two main branches - Differential Calculus and Integral Calculus. Both branches are related to each other by the fundamental theorems of calculus. They make use of the fundamental notions of convergence of infinite sequences and infinite series to a well-defined limit. Calculus as we see it today, was initiated by Isaac Newton and Gottfried Wilhelm Leibniz, back in the 17th century.
Calculus is a vast topic but this is only an overview refresher. If you are interested, you can check out the textbooks that cover the topics in detail Thomas' Calculus by George B. Thomas and Joel Hass is one such interesting text.
The three main aspects of Calculus are :
The subject is vast and interesting. Each aspect of calculus is immensely useful in different branches of science, engineering and finance.