What the ISNUMBER function does is to check to see whether the data in a certain cell is a number or not. If the data is a number, a value of TRUE is returned by the function. If it is not a number, or the cell is empty, a FALSE value is returned.
This function, like others in its group, is most often used to correct errors in a spreadsheet. In large spreadsheets, finding an error in a function or formula that uses the wrong type of data, can be difficult to do. The information functions in Excel are designed to make the job of finding such problems easier.
To find out all the details of using this function read the short article on the Excel 2007 ISNUMBER Function.