From the xl help - always a good place to start
If you perform a task repeatedly in Microsoft Excel, you can automate the task with a macro. A macro is a series of commands and functions that are stored in a Microsoft Visual Basic module and can be run whenever you need to perform the task.
For example, if you often enter long text strings in cells, you can create a macro to format those cells so that the text wraps.
Recording macros When you record a macro, Excel stores information about each step you take as you perform a series of commands. You then run the macro to repeat, or "play back," the commands. If you make a mistake when you record the macro, corrections you make are also recorded. Visual Basic stores each macro in a new module attached to a workbook.
Making a macro easy to run You can run a macro by choosing it from a list in the Macro dialog box. To make a macro run whenever you click a particular button or press a particular key combination, you can assign the macro to a toolbar button, a keyboard shortcut, or a graphic object on a worksheet.
Managing your macros After you record a macro, you can view the macro code with the Visual Basic Editor to correct errors or change what the macro does. For example, if you wanted the text-wrapping macro to also make the text bold, you could record another macro to make a cell bold and then copy the instructions from that macro to the text-wrapping macro.
The Visual Basic Editor is a program designed to make writing and editing macro code easy for beginners, and provides plenty of online Help. You don't have to learn how to program or use the Visual Basic language to make simple changes to your macros. With the Visual Basic Editor, you can edit macros, copy macros from one module to another, copy macros between different workbooks, rename the modules that store the macros, or rename the macros.
Macro security Excel provides safeguards against viruses that can be transmitted by macros. If you share macros with others, you can certify them with a digital signature so that other users can verify that they are from a trustworthy source. Whenever you open a workbook that contains macros, you can verify their source before you enable them.