Excel understands two different styles of referencing for cells and ranges. These are described in the table below :
A1Style References
The A1style is the most common form of Excel referencing, and is the default style.
This style of referencing is made up of a letter and a number, which represent the column reference and the row number, respectively. Eg.
B2
C5

R1C1Style References
The R1C1style of referencing is made up the letter R followed by a row number
and the letter C followed by a column number.
The row or column number of the R1C1 style reference can be encased in square brackets, if you want the row or column number to be measured as a shifted number of rows or columns from the current cell location. If the row or column number is omitted, this tells Excel to look in the current row or column Eg. If the current cell is cell C5, then :

Note that you need to tell Excel which style of referencing you are using. This is set by checking, or unchecking, the option 'R1C1 referencing style' in the Excel options menu. This option can be found:
In Excel 2010 :  In the File menu, under Options→Formulas 
In Excel 2007 :  In the main Excel menu (accessed by clicking on the Excel Logo on the top left of the spreadsheet), under Excel Options→Formulas 
In Excel 2003 :  In the Tools dropdown menu, under Options→General 
By default, Excel uses the A1 style of referencing, and the columns of your spreadsheet are labelled with letters. However, if you select the R1C1 option, you will notice that the top of your Worksheet columns display numbers, instead of letters.