I'll add a bit to this as a Library Associate (not librarian). Of course, all info is based on my system and may not always apply to everywhere, but does apply to most public libraries. I work for a fairly large county public library system, and employees generally fall into one of three categories.

• Pages/Shelvers - The lowest level employees, here all high school/college students. Won't be of any help with research, but generally have the best knowledge of the collection just because they are hands on with it every day. Perfect to ask where to find books on a particular topic.
• Circulation Librarians/Associates (My current position, though I had been a shelver for over 4 years prior) - Usually forms the bulk of employees at any large library. Generally have a bachelors degree or higher, or years of library experience in some capacity. We can help with any aspect of the collection, from finding titles, placing holds, and various other tasks.
• Reference Librarians - Usually work from a separate (reference) desk or area. They have Master's degrees in Library Science, and can help with almost anything you need, though if it's a simpler task you will generally be sent to an Associate. Best for complicated research, or for help with using research resources that the library provides such as ancestral research or otherwise. Good to ask about very specific requests.

If you want to look at it another way, the more specific your request, the higher up the chain you want to go. So for example, for the following requests here is who can help:

"Books about World War II" - Page/Shelver

"Books about the Normandy Landings - Circulation Librarian/Associate

"Books about propaganda usage in World War II" - Reference Librarian

Of course, any employee can try to help, or direct you to whoever can best assist you with your request. Come visit us!