A shared library is a collection of programming code that multiple programs can use simultaneously. Instead of each program having its own copy of the code, they can access the shared library, which saves memory and disk space. This approach allows for easier updates, as changing the library will automatically affect all programs that use it. Shared libraries are commonly used in various operating systems, such as Linux and Windows. They typically have file extensions like .so for shared object files in Linux and .dll for dynamic link libraries in Windows. This system promotes efficient resource management and faster application development.