The North was the industrial part of the country. It depended on its' factories for most of it's revenues. The factories where mostly textile factories that processed cotton into cloth, then clothing, and processed and made other products. Between 1790 and 1860, commercial agriculture replaced subsistence agriculture in the North, and household production was replaced by factory reproduction. Massive foreign immigration from Ireland and Germany greatly increased the size of the Northern cities. From 1960 to 1965 the U.S got about three hundred thousand immigrants from Ireland and four hundred thousand immigrants from Germany (Bailey: 450). The massive immigration also meant cheap workers for the factories.
In the South, slavery impeded the development of industry and cities. It also discouraged technological innovation. The South did not industrialize like the North did they remained reliant on agriculture. By remaining reliant on agriculture like cotton and tobacco, they were also indirectly relying on the North. Everything the South grew they would ship to the North to be processed. They would send the cotton over to be processed into thread and then from thread into cloth and from cloth into clothes. After the process is done it is shipped around the world.
This "balance" was disrupted by political and economical differences between the North and South. The North was pushing for a protective tariff in order to help keep most of the business within the country not in imports. The South didn't want any tariffs on bought goods. Another factor