Mills changed the work from manufacturing to mechanized and specialized. The work that was made entirely by one person now was broken down into specific tasks, many machines took the labor from humans and people had usually repetitive work.
But this caused also many people to move from the country to the cites because the Mills needed hundreds of workers, they usually lived in company towns. This made the North develop faster than the South and have big cities with a big population in contrast with the Southern states.