Excel treats disk files in a number of ways, depending on the type of file and the version of Excel being used. For example, Excel 97 and earlier versions treat .xls files as read-only, while Excel 2000 and later versions treat them as read/write.
When a workbook is first opened, Excel checks to see if the file is already open in another instance of Excel. If so, Excel opens the workbook in read-only mode. If the file is not open in another instance of Excel, or if the other instance is using an earlier version of Excel, then Excel opens the workbook in read/write mode.
When a workbook is saved, Excel first saves a copy of the workbook with a temporary file name. Once the copy is successfully saved, Excel then deletes the original file and renames the copy to the original file name. This process helps prevent data loss if there is a problem saving the file.
If you try to open a file that is already open in another program, you will get an error message telling you that the file is already open. You can still open the file by using the Open dialog box (click File > Open), but you will not be able to edit it until you close the other program.