A function is a type of formula built in to Google Spreadsheets. You can use Google's built-in functions instead of writing complex formulas in your spreadsheets; you can also include functions as part of your formulas.
Functions simplify the creation of complex formulas. For example, if you want to total the values of cells B4 through B7, you could enter the following formula:
Or you could use the SUM function, which is built in to Google Spreadsheets. The SUM function lets you total (sum) a column or row of numbers without having to type every cell into the formula. In this instance, the formula to total the cells B4 through B7 could be written using the SUM function, like this: ...