Q. What is the difference between the library catalog and SLUth?

The best place to start your research is SLUth Search Plus. SLUth includes articles published in scholarly journals, magazines, and newspapers as well as books, ebooks, and government information. It's very multidisciplinary in scope, so you can use it to find credible sources of information on any topic. It includes about 100 of the SLU Libraries 400 (or so) subscription databases. So, you're not searching everything the SLU Libraries have, but you're searching a lot.  

If you're specifically looking for books in the SLU Libraries you'll find that searching the SLU Libraries Catalog is easier than searching SLUth Search Plus. The catalog is an inventory of materials that are available in and via the SLU Libraries, including books, journals, magazines, newspapers, and media materials. What it doesn't include is individual articles. You'll need to use SLUth Search Plus for that.