An Excel function is a predefined routine. These functions are available when the user accesses the Formulas tab and then clicks Insert Function. A dialog box appears with all functions, but the user can filter by category or recently used. Some functions are common math functions such as SUM. In contrast, others perform an action on cell elements such as TRIM. These functions can be used as building blocks in Excel formulas and also nested. The user does not need to know the code behind the routine, but merely the function name.