Source : Free On-Line Dictionary of Computing

Dynamically Linked Library
     
         (DLL) A {library} which is linked to {application
        programs} when they are loaded or run rather than as the final
        phase of {compilation}.  This means that the same block of
        library code can be shared between several {tasks} rather than
        each task containing copies of the routines it uses.  The
        executable is compiled with a library of "{stubs}" which allow
        {link errors} to be detected at {compile-time}.  Then, at {run
        time}, either the system {loader} or the task's entry code
        must arrange for library calls to be patched with the
        addresses of the real shared library routines, possibly via a
        {jump table}.
     
        The alternative is to make library calls part of the
        {operating system} {kernel} and enter them via some kind of
        {trap} instruction.  This is generally less efficient than an
        ordinary {subroutine} call.
     
        It is important to ensure that the version of a dynamically
        linked library is compatible with what the executable expects.
     
        Examples of operating systems using dynamic linking are
        {SunOS} (.so - shared object files), {Microsoft Windows}
        (.dll) and {RISC OS} on the {Acorn} {Archimedes} (relocatable
        modules).
     
        (1995-12-12)