Calculus, which is usually considered to be a hard course, is actually not that difficult, especially for the ones who was good at algebra at school. Calculus was developed out of a need to understand continuously changing quantities. The study helps to understand something by looking into separate small pieces of it. Many branches of science are based on calculus, such as physics, astronomy, engineering and others.

There are two main branches of calculus.

The first is differentiation (or derivatives), the part concerned with continuous change and its applications. It provides us with a very powerful tool for understanding the behavior of mathematical functions. Put simply, differential calculus breaks something into pieces to learn more about its way of changing.

The second branch is integral calculus. Integration is a process which, simplistically, resembles the reverse of differentiation. So, small numbers (or pieces) are added to understand how much there is in general.

More Info: en.wikipedia.org