Your computer is like a city: many streets, shops in every street, many choices. In the beginning it is not obvious to know where you are and how to go where you want to be.
Compass
A few things on your screen never change. They form your compass, from which you can always start.
- The cursor:

The cursor is always on the screen and always moves with the mouse. If you cannot find the moving cursor when you move the mouse around, your machine has a problem! Do not confuse the terms cursor and insertion point. The cursor is usually an arrowhead, but it can take other forms.
- The menu bar:

The menu bar is always at the top of the screen. Each word in the menu bar is the title of a pull-down menu. At the extreme left is the Apple menu, represented by a small Apple logo. The next menu is the application menu: its name changes with the active application. The following menus also depend on the active application. At the extreme right is a set of icons for quick access to some system functions.
(in a few cases the menu bar may be hidden: e.g. when you watch a DVD full screen)
- The Apple Menu or System menu:
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The system menu is for operating system functions:
this is where:
- you can find out about the qualities of your Mac
- you set your personal preferences (in the System Preferences) that will apply thoughout applications
- you select the network location
- you find the list of objects you used recently
- you abort an application that misbehaves (Force Quit)
- you put your machine to sleep, restart it or shut down.
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- The Dock:

is the set of icons showing your running applications, perhaps some documents, and the wastepaper basket. Use it to organise shortcuts to most used items.
Bearings:
- The application menu:
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The application menu is always the first menu after the system menu:
this is where:
- you can see which is the active application: its name is the name of the application that is currently listening to your commands.
- you find out about the active application: which version it is, which company made it, etc.
- you set the preferences that will apply to that application
- you can temporarily hide all windows of the application
- you can quit the application.
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All menus to the right of the application menu will depend on the active application. The first two are usually called the "File" and the "Edit" menu. However, there are applications that do not have a File or edit menu.
Disaster
On extremely rare occasions a grey curtain will come down over your screen

and you will see this message in the centre:

the "kernel panic" message
The phenomenon is called a "kernel panic" and it means OS X has stopped. It should in principle never happen, but no operating system is perfect. A kernel panic is effectively a system crash (the kernel is the inner part of the system). There is nothing you can do apart from starting the machine again. All your work that was not saved is lost.
You should always note down what you were doing when the kernel panic happened. If you have frequent kernel panics (say more than once a month) you should try to diagnose the problem or you should consult someone with more system knowledge, and have your notes available.
System crashes may leave the information on your discs in an incoherent state. After a kernel panic, the system will run a thourough check of the discs to look for and repair any such incoherences. It may therefore take longer than usual to start up after a kernel panic.