Mustafa,
It is something which I have not found much about on the web.  Partly
because it is so easy once you are shown, by far the best way is to ask
someone who can actually show you as you work.

All formulae can work equally well with cells from the worksheet where the
formula is, another worksheet in the same workbook, or another workbook.

There is a bit here on links
http://www.tutorialbox.com/tutors/off2000/excel/functions.html

You can practise by building a little formulae.

Have a workbook with 2 or more worksheets.
press "=" then click on an other worksheet, put your cursor on a cell in
this workbook, press return.
You now have a cell linked to another workbook.

Have 2 or more workbooks open.
press "=" then choose window from the menu, click on another workbook name
to go to it.
Put your cursor on any cell in that workbook, press return.
You now have a formula linked to another workbook.

Note in the first example the formula is relative by default, and in the 2nd
it is absolute, see the link for explanations of these terms.

When you build any formula, however complicated you can switch to another
workbook or worksheet and reference a cell in them.

If you have a specific requirement for linking post back with details, it is
difficult to give advise with such a general request.