Excel formulas are used to process simple or complex equations and output the result to a specific cell within a worksheet.
(There is an exception to this where the WorksheetFunction class is used in VBA programming, however VBA is out of scope for this tag - please refer to the excel-vba or vba documentation for VBA related articles.)
Formulas can be used to convert, manipulate or evaluate many different kinds of expressions using all different kinds of data. There are however some restrictions around formulas and what they are able to achieve.