In computing, a library is a collection of similar objects that are stored for occasional use - most frequently, programs in source code or object code form, data files, scripts, templates, fonts, and physical storage units such as tape cartridges. Here are some common types of libraries.
1) A program library is a collection of (usually) precompiled, reusable programming routines that a programmer can "call" when writing code so that the programmer doesn't have to write it. A dynamic link library (DLL) is one type of program library. Another type of program library is a class library, whose stored routines are class definitions in object-oriented programming (OOP). Graphical user interface (GUI) components such as scroll bars, buttons, and windowing routines are generally stored in a class library.
2) A storage library is a collection of physical storage media such as tapes or disks and a way to access them. A tape library, for example, contains tape cartridges and a mechanism that moves them into and out of the drive(s) where their content is read or updated.
3) A data library is the area of a data center (a centralized area housing computer systems and equipment) where storage media are archived. Online service providers also sometimes refer to a directory on a server containing files for downloading as a data library.
4) A virtual library is simply the online version of the traditional library. Books and documents are made available over the Web, and may be read on line or downloaded.