shared libraries are libraries that are shared by one or more executing programs. shared libraries is a term normally used in Linux communities while a similar kind of library is known as Dynamic Link Libraries in Microsoft Windows. This tag is used when a question is about how to build, install, deploy, or use shared libraries. This tag should be used with a product specific tag and OS specific tag.
A shared library is a form of a library which is shared between multiple executing programs so that there is only one instance of the library in memory. The primary purpose of a shared library is to reduce the amount of memory used by programs that using the library as a single instance is shared among multiple programs rather than each program having their own copy.
There are other advantages to using a shared library in addition to memory reduction. A shared library can be updated in such a fashion that programs using it do not need to be rebuilt.
This article, Shared Libraries from The Linux Documentation Project describes what shared libraries are and how they are used including setting up the execution environment and installing shared libraries.