Substitution is an integration technique used to simplify complex integrands by replacing variables with new ones that make integration easier.
U-substitution is a specific type of substitution where we let u represent an expression within the integrand.
Integration by parts is another integration technique that involves breaking down an integral into two parts and applying a specific formula.
Trigonometric substitution is a method used when dealing with integrals involving square roots of quadratic expressions or trigonometric functions.