Library of Congress Classification (LOC) is a way of organizing things based on its specific topic. It is a series of letters and numbers arranged in particular ways, starting with major categories and furthering breaking them down into subcategories, sub-subcategories, even sub-sub-subcategories. LOC is one of several classification systems used in libraries, like the Dewey Decimal Classification (DDC), the Universal Decimal Classification (UDC), and Bliss Bibliographic Classification (BC). However, you will see that most libraries use either DDC or LOC. Typically, you find college and university libraries using LOC and the public and school libraries using DDC.
It is not easy to master but, with a little practice and help from your friendly Lindsey librarian, you can find just about anything that you want or need to learn about!