I was going through some exercise given in my school.I have read the C book but I am not able to understand some part.
That is static variable.
What exactly is a static variable and what does it do?
I saw in Linux Kernel Programming
external and exported are also some thing.
I am not getting the difference between static,EXPORT_SYMBOL,external variable types.
If I make a kernel module then how will I make sure that my variable is visible to the kernel.
Is this what it is all talking about?
I have checked this page
http://en.wikipedia.org/wiki/Static_variable

You're looking in the right places. However, you probably need some examples.

So, if in my module (to make it simple: one file) I declare a variable and make it static, it'll be available only to code from this file, not from the rest of the kernel. Any other variable will be, but it isn't a good idea to access it directly from the rest of the kernel, because the module may be unloaded at any time.

External is just an indication for the compiler that the variable will be defined somewhere else.

EXPORT_SYMBOL is a macro and may have different meaning, depending on the options. Generally it makes a kernel variable visible from modules. Look how it is defined.

The keyword 'static' has more than one meaning in C, which can lead to confusion.

When you declare a variable static inside a C function, it means that the value will be preserved between function calls, because its storage class is static (typically this means that it is implemented by storing it in a data segment, rather than on the stack).

However, for global variables declared outside a C function, which you would expect to be of a static storage class anyway, it also means that the scope of the variable is limited to the file that contains it. By default, if you do not declare a global variable as static, it will be available to other modules that are linked together with the one that declares the global variable.

The extern declaration on a global variable simply means that the variable is created elsewhere; you have to declare it so that the compiler knows its type, but it is allocated space elsewhere (and it is up to the linker to complete the compilation by inserting the actual address).