Integration by parts is a technique used in calculus to integrate products of functions. It is based on the product rule for differentiation and is expressed by the formula ∫u dv = uv - ∫v du, where u and v are differentiable functions. This method is particularly useful when one function is easily integrable and the other is easily differentiable. To apply integration by parts, you first choose which part of the integrand to assign to u and which to dv. After differentiating u to find du and integrating dv to find v, you substitute these into the formula. This process can simplify complex integrals, making them easier to solve.