The major change in American business during this time was the fact that companies started getting bigger and bigger. By the end of the century, the US was in the middle of the era of "robber barons" and the monopolies or near monopolies that they and their companies enjoyed. Men like Rockefeller and Carnegie exemplified these changes with their creation of huge oil and steel companies.
The two men also later exemplified the idea of the social gospel. Both men gave huge amounts of money later in life to charitable causes. This showed their dedication to the idea that the rich had a duty to spend their money to improve their society.