Traditionally, libraries have been thought of as collections of (old) books in single places for easy access by a given - frequently small - community of (frequently old) library users, and presided over by a (frequently old) professional officer known as a "librarian" - whose job it is (ideally) to enable library users to find the information they want within the collection. Such collections are often designed to catalogue specific areas of human knowledge (e.g. legal knowledge, medical knowledge, historical knowledge, etc.) as represented by the books they contain. Public (County, Municipal, State, National) libraries, however, cannot limit the natures of their collections in this way, and must often collect as much information of any type as possible, to satisfy the wildly variable information demands of their user communities. The books themselves are made available for borrowing by the community of users.
There are numerous problems with this traditional definition in a modern context. First, library collections are no longer predominantly comprised of books: they also frequently contain maps, video tapes, computer software, newspapers in print and on microfilm, magazines, and Internet access terminals. Second, library collections are no longer even often presided over (if they are presided over at all) by a "librarian", but more and more frequently by an "information manager", "information consultant" or "information assistant". In spite of such changes in the name of their profession, however, those who preside over libraries usually finish up behaving like librarians: putting a lot of the collection's items away once they are returned, chasing those who haven't returned them, and saying "shhh" a lot. Third, the concept of the library in a "single place" has undergone much revision as the amount of printed information available in the world, and the ability to gain access to it via communication tools such as the Internet, has expanded.The proliferation of access to such communication tools means that the connection of larger communities to a given library collection is made easier, and hence the library user community becomes larger, which means, effectively, that there are fewer books on a given subject to go around. It no longer makes sense to try to house the whole of human knowledge in one place because (again with the advent of communication technology which can enable its transmission) the volume of information in circulation is simply too large, and is too frequently updated to be collected systematically. Small "sub-collections" which form parts of larger library groups - each library within a group collecting items on different subjects - have become very fashionable as a result.
In practical terms, all of this means that the service purpose of a library - to provide you with the information you want at minimal cost for as long as you should really need it - is often not achieved. Even if they have a given book on a given subject, somebody most likely has it out. It is even more likely, however, that the library you visit will simply not have the book, although "somebody else in the network" probably will. It also means that, no matter how much money the library is given to achieve a given purpose in the collection of information, it can usually not achieve it. For these and other reasons, librarians are frequently characterised as unhappy professionals.