As well as adding obvious, visible functions like that, though, developers wrote extensions that added more esoteric functionality to the OS, usually in the interests of helping their applications run better. These extensions usually install themselves when you install an application, often without you even being aware it's happening. This has led to an explosion in the number of extensions the average user has crawling about in their system.
As I said, this was pretty neat in the early days. It's not so neat now. These days, rampant extensions tend to slow things down as much as they used to help. As well as adding considerably to the amount of time it takes to start up your machine, they consume precious processor time and RAM. They are also the single main cause of instability in the Mac OS (see "Conflict resolution"). This is why Apple has redesigned Mac OS X to do away with extensions.
Until you make the switch to OS X, though, it is still possible to operate your Mac more efficiently, by juggling around extensions and making sure you're running only the extensions you need to do what you're doing. The way to do this is with the Extensions Manager.
Using Extension Manager
Extensions Manager is a control panel, which you can access either via the Apple menu or by holding down the space bar when you start up your Mac. When you open Extensions Manager, you're presented with a group of folders representing your Control Panels, Extensions, Startup Items, Shutdown Items and other miscellaneous items in the System folder that don't quite fit into any of the other categories. To get more details on the contents of these folders, just click on the little triangles to the left of each one. At the bottom of the control panel is another blue triangle labelled "Show item information". It's a good idea to click on this, as it will offer some guidance when trying to optimise your system.
At the top of the panel is a pull-down menu listing the sets of extensions that have been created. By default, this includes a "Base" set for the minimal set of extensions you require to run your system, and an "All" set that includes every extension Apple provides with your system software, some of which are not required to run your system (not everyone needs ColorSync, for instance).
You can, of course, run the Mac OS with no extensions at all, by holding down the shift key when you start up. It's useful to do this sometimes (see "Conflict resolution"). However, with no extensions, you'll lack some of the obvious niceties of the Mac such as the Appearance manager (no desktop pictures) and cascading menus. So, this option is only to be resorted to in emergencies.
Adding Your Own Extensions
When creating your own sets of extensions, it's a good idea to start with the basics. Select the "Base" set from the pull-down menu, and click on the button labelled "Duplicate Set". Give your new extension set a name, such as "Games set" or "Publishing set" or whatever, depending on what you want to optimise your Mac to do. Your new set then becomes the currently selected set.
The next thing to do is to add the extensions you require to your new set without adding any unnecessary clutter. Click on the triangle next to "Control Panels" to view all the control panels you have installed. Panels which are included in your set are indicated by a cross in the checkbox to the left. To add an item, simply click on this box. To find out whether you require a particular control panel, click once on it, and it will appear in the panel below, where it says "Show item information". In most cases, this will reveal the developer of the control panel and its version number. In some cases there will be a brief description of what the item is and what it does. If it's applicable to what you need to do, add it to your set.
Another way to find out whether or not you need a given control panel or extension is to look in the column labelled "Package". Click on the column heading, and Extensions Manager will sort all items into groups that go together, e.g., all the bits of QuickTime will be grouped together so that they can be activated at once. Likewise, if your graphics accelerator requires a group of extensions to be installed, they'll be easy to find.
Of course, some applications (QuickTime, for instance) require "control panels as well as extensions. If you want to group all items in packages, regardless of whether they are control panels or extensions, you can choose "view as packages" from the "View" menu. This has the effect of not only grouping items in packages, but also making it possible to activate or deactivate an entire package with a single click, as well as being able to get information about an entire package at once. If you find yourself getting lost in the process of adding extensions and control panels, click on "Revert", and the set will go back to the state it was in when you started.
Using Your New Extensions
Once you've worked out what extensions and control panels you need to run your Mac efficiently, close Extensions Manager to finalise the set you've created. When you do so, you may see a series of dialogue boxes indicating that multiple versions of some items have somehow made their way onto your Mac. This often happens if, for instance, an application installs an earlier version of some extension than the one you already use. And it can cause problems - I can't count the number of crashes I've had as a result of multiple Sound Managers lurking about. In this case, check the version numbers in the dialogue boxes and delete the older files.
You must restart your machine in order to activate your changes. You can always go in later and change a set if you find that something doesn't work, but you will have to restart your machine every time you do so. Also, it's worth remembering that you can open the Extensions Manager upon startup by holding down the space bar. Then, just select the set you want to use and click "Continue" for a lean, mean, streamlined Mac.
More information about your extensions (especially ones that don't provide information in Extensions Manager) can be gleaned from Dan Frakes's InformINIT (http://mc04.equinox.net/informinit), a third-party shareware program that lists detailed info on literally hundreds of extensions and control panels.
As well as making your Mac run more efficiently, Extensions Manager can be used to resolve conflicts that make individual applications less stable. Very often, when an application crashes, the fault is not with bad programming, nor with the Mac itself, but with a simple incompatibility between two or more extensions that you have installed.
If one of your applications is misbehaving, open up Extensions Manager and activate only the extensions that you know you require for the application you wish to run (the "view as packages" feature can be very useful for this) and deactivate pretty much everything else except the basics. Then, once you're sure the application can run well, go back into Extensions Manager and activate other extensions, restarting every time. When you activate something and the application in question no longer runs well, you've found the culprit. You can then create an extension set that includes the extensions you need for that application, and excludes all others.
A quicker way to do the same thing, if you don't mind spending a bit of money, is to install Casady and Greene's Conflict Catcher (www.pica.com.au), a third-party application that replaces the Extensions Manager and adds functions such as automatically testing sets of extensions for clashes.