Vertical integration allowed a company to control the businesses related to supplying the product for that business. For example, Andrew Carnegie developed vertical integration related to his steel company so that he also controlled mines from which he could extract iron, and he bought railroads on which to transport his steel. This made his process cheaper, and it allowed him to develop an even vaster monopoly. Today, Apple practices vertical integration, as it controls many processes related to producing and selling its products, including retail stores.
Horizontal integration refers to the process of controlling an entire industry at the same point in the supply chain. For example, today, many media companies own television, radio, print, and internet companies. This allows them to repackage content for different media without needing to gather it again. Both of these processes were significant because they fostered the increased growth of monopolies during the Gilded Age in American history, leading to great power concentrated in the hands of a few business owners.