Calculus is a branch of mathematics that focuses on the study of change and motion. It is divided into two main areas: differential calculus, which deals with rates of change and slopes of curves, and integral calculus, which focuses on accumulation and areas under curves. Calculus uses limits to analyze functions and solve problems related to motion, growth, and optimization. It is fundamental in fields like physics, engineering, economics, and biology. Key concepts include derivatives, integrals, and the Fundamental Theorem of Calculus.