YA Librarian, or teen librarian, is a librarian

Librarian

A librarian is an information professional trained in library and information science, which is the organization and management of information services or materials for those with information needs...

 who works primarily with young adults

Young adult literature

Young-adult fiction or young adult literature , also juvenile fiction, is fiction written for, published for, or marketed to adolescents and young adults, roughly ages 14 to 21. The Young Adult Library Services of the American Library Association defines a young adult as "someone between the...

, generally defined as between the ages of 12 and 18. A YA librarian's duties may include reviewing and ordering teen books, magazines, and media; planning library programs for teens; providing reference services to teens and other library patrons; visiting middle and high schools; conducting tours for school and community groups; teaching teens how to use the library's catalog and databases; doing outreach to teens in the community; and much more.

YA librarians work primarily in public libraries

Public library

A public library is a library that is accessible by the public and is generally funded from public sources and operated by civil servants. There are five fundamental characteristics shared by public libraries...

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Web sites for teen librarians include Teenlibrarian.com and Alternative Teen Services.