Adding comments to an excel spreadsheet can be a great way of including information about the data in a cell. Cells should normally only include one type of information – say a number. (See my previous post on Using Excel for more about this.) If you need to include a note about the data in a cell you can either create a new column for notes or you can add a comment. The comment tools are found on the Review tab of the ribbon.
Full instructions for using comments are here: http://office.microsoft.com/en-nz/excel-help/annotate-a-worksheet-by-using-comments-HP010342170.aspx