Library users may wish to understand the letters and numbers on the spines of books and on other items (CDs, maps, etc.) in the library collection. These numbers are Call Numbers. Melville Dewey created the Dewey Decimal Classification System and he pretty much used numbers on the first line. Perhaps the words call numbers persist because of the popularity of his system.
Anyway, the call number is a unique way of identifying the object (book, CD, etc.) and also its place in the collection. Before public libraries became wide-spread, libraries did not always allow users/patrons to go to the shelves to obtain the desired item. The staff at a desk received the request (call number) from the patron and then went to the shelf to obtain it. As you can see, the call number has a long-lived history of importance in the library.
So, what is the object of the letters and numbers? If you are interested in knowing about the Library of Congress Classification System, click here for a website showing the letters of the alphabet with their accompanying subjects. Pretty much, any classification system is a method of organizing knowledge. This concept is old. But as we have come to the present century and with the proliferation of print media and now electronic media, the idea of organizing knowledge has become daunting. The Library of Congress is the agency that deals with all of this growth and expansion and provides a place in the classification system for “new knowledge” or for new things presented in different ways (media).
Not all of the letters of the alphabet have been used. For example, there are no Is nor Os, nor Xs. After the first letter, the classification begins to expand thus: A, AA. AB, AC, and so on. This occurs for most of the letters. You may not realize that when you learned the alphabet in kindergarten and first grade that this knowledge might become useful!
After the letters, the subject of that area is further identified by the numbers, which ascend as usual (1….10….100….1000) and maybe a decimal point with numbers after. (Thanks, Melville, for that wonderful idea.)
Here is a call number:
Hv 6250.3 U5 L357 2014
The online catalog has the availability and the location of this book. Most of the items that can be taken out (circulated) are housed on the lower level and circulate for 14 days with a possibility of renewal. The collection is divided with the call numbers A-K located on the west side of the room and the call numbers L-Z located on the east side of the room. All media and books are interfiled on the shelves.
The end panels of the stacks on the lower level have the range of call numbers for each section. 
Still stumped? Go to the Information/Reference Desk (circle-shaped desk) and ask a librarian to help you.