What is an extension?

An extension is a small file that changes how your computer operates. For example, the QuickTime extension gives your computer the capability to play digitized video and audio files. Unlike a control panel, you typically do not have any direct interaction with an extension. An extension makes the appropriate changes when you start your computer and remains active until you shut down your computer. Extensions must be loaded into memory during computer startup. Therefore, when you place (or install) a new extension into your extension folder. it has no effect until you restart your Macintosh. Extensions with the letters "lib" in them are typically code libraries that may be used by multiple applications or system components. Turning off extensions or libraries may have unexpected effects. In general, you shouldn't turn off any extensions unless you know for certain what they're used for.

The contents of your Extensions folder may vary depending on the type of computer you have. You might also have extensions that were added by third-party applications. You can use the Extensions Manager control panel to turn off any extension you don't need. However, Extensions Manager will only list those files that have an INIT resource, or files that are set to be listed regardless of whether they have INIT code in them or not. In other words, Extensions Manager does not show all extensions and control panels.

The Extensions folder inside the System 7.x and 8.x System Folder contains "system extensions" (previously known as "INITs"), and miscellaneous software additions to your Macintosh computer, such as printer drivers and network drivers. The Extensions folder is the first place the System looks when loading system extensions at startup.

What is a control panel?

A control panel is a small piece of software that allows you to change your computer's settings to meet your needs. For example, you can change the pattern or color of your computer's desktop using the Desktop Patterns control panel.

You can use the Extensions Manager control panel to turn off any control panel you don't need on your computer.

Depending on the type of computer you have, the contents of your Control Panels folder may vary. You might have control panels that were added by third-party applications, and you don't need all the control panels described below. For complete instructions on using control panels, see Macintosh Guide, available in the Guide (question mark) menu when the Finder is the active program.

What is Shared Library file?

Shared Library Manager
Apple Shared Library Manager (ASLM) extension is a piece of code that lets other programs share common code. Historically each application was required to link in any necessary library modules at compile time. The shared library manager lets any common code be loaded into memory only once by all running applications and then shared by others who also need to make the same calls thus reducing the memory foot print for application.

ASLM ships with several Apple as well as some third-party products. Any application that requires ASLM ships with a copy of the application. ASLM maintains the relationship of applications to shared libraries at the system level. 

Shared Libraries
A shared library is a library of functions or classes (for C++ programmers) that are compiled, linked, and stored separately from the clients (applications) that use them. By accessing the functions or classes that are stored in a shared library, a client (application) can call functions that are not part of its executable code. Furthermore, functions or classes that are stored in a shared library can be called by different applications that are running at the same time.

Because shared libraries can contain shared code and are loaded and linked at run time, they save enormous amounts of RAM and disk space. Shared libraries eliminate the necessity for keeping multiple copies of code in memory when multiple applications use the same code. Shared libraries help software developers design independent, modular, compact libraries that applications can share. It also helps software designers develop their products faster, and it makes the products easier to improve and maintain.

A shared library file is a binary file that can contain object code for functions, classes, methods (member functions), data, and resources. A shared library file can contain one or more shared libraries. When a shared library file is made available, developers can share, and dynamically link with, the code stored in the shared library.