Until VoG comes along to give a detailed answer, this may help from the excel help file.
Merge data from several workbooks.
You can link workbooks from several users or departments and then integrate the pertinent data into a summary workbook. That way, when the source workbooks are changed, you won't have to manually change the summary workbook.
Formulas with links to other workbooks are displayed in two ways, depending on whether the source workbook— the one workbook that supplies data to a formula— is open or closed.
When the source is open, the link includes the workbook name in square brackets, followed by the worksheet name, an exclamation point (!), and the cells that the formula depends on.
For example, the following formula adds the cells C10:C25 from the workbook named Budget.xls.
When the source is not open, the link includes the entire path.