Interlibrary Loan (ILL) is the term used for the sharing of library resources (books, articles, DVDs, etc.) between libraries, for the benefit of their individual users (patrons).
Somewhat early in the history of libraries, librarians realized that they could not possibly purchase all of the resources all of their patrons would like to use. They also realized that other libraries were likely in the same position, so they started requesting to borrow other libraries' materials in exchange for lending their own items.
Today, most libraries belong to at least one network comprised of many libraries which have agreed to share their resources with each other. Central Penn belongs to Access PA, so in exchange for lending our materials to other libraries, we can borrow materials from most of the other libraries in the state--that includes Penn State and Temple University's libraries, among others.