Public and School libraries have certification requirements for some staff members.
From the PA Department of Education site:
All public library directors in libraries receiving state aid are required to have a certification level based on the population of the service area of the library where they are employed. The certification levels are: library assistant, provisional librarian, and professional librarian.
Pennsylvania teaching certificates with endorsement in school librarianship may be issued to graduates of Pennsylvania colleges and universities who have [met certain requirements].
Most professional-level librarian positions require an ALA-accredited master's degree in library and information science. Non-MLS positions are usually paraprofessional-level and in most libraries pay less, although some smaller or rural institutions are staffed mainly or entirely by non-MLS workers, and some of these are staffed by one person.