Internal audit is loosely defined as “control and operational auditing”. In many companies, the focus is to manage the SOX requirements, or the equivalent controls over financial reporting in a non-public company. In other companies, like my own, there is a separate group that manages SOX, so the focus is more on operational audits (strategy, mgmt reporting, products, real estate, basically anything else).
Every company is different (with a fair amount of overlap), but auditing a services company would have little transferable skills to some other major sectors. Could be great if you like the industry and would like to stay and grow, but if you find you don't love it, it's best to leave within a few years.
Ideally, we all know what our career goals are, but many (myself included) kind of go with the flow and try new things.
If you don't know what it's about, then that's what you should learn when applying and/or during an interview. That's the time for you to get the detail on what you're actually going to do!