Contouring is a technique used in makeup to change facial features by applying highlighter, shader, and blusher. Highlighter is lighter than foundation and emphasizes areas, shader is darker and minimizes areas, and blusher adds warmth. Contouring can correct eyes, faces, chins, jaws, and noses. It is important to determine a client's face shape to see if contouring is needed, with the goal being an oval shape. Different face shapes like round, diamond, and square require highlighting and shading certain areas to balance features and achieve a desired oval shape.