Integration is a major part of calculus, related to differentiation by the fundamental calculus: If $f$ is the derivative of $F$, then $$\int^b_a f(x)dx = F(b)-F(a)$$
Integration is often harder than differentiation. Several techniques are developed to help integrating, these include integration by parts, integration by substitution, trigonometric substitutions and partial fractions.
Integration can be used to find the area under a graph and finding the average of the function. It can also be used to compute the volume of certain solids.