In simplest terms, a function is a predefined formula. Many Excel functions are shorthand versions of frequently used formulas. For example, the SUM function adds a series of cell values by selecting a range. Compare the formula =A1+A2+A3+A4+A5+A6+A7+A8+A9+A10 with the formula =SUM(A1:A10). The SUM function makes the formula a lot shorter, easier to read, and easier to create.

Some Excel functions perform complex calculations. For example, using the PMT function, you can calculate the payment on a loan at a given interest rate and principal amount. All functions consist of a function name followed by a set of arguments enclosed in parentheses. (In the preceding example, A1:A10 is the argument in the SUM function.) If you omit a closing parenthesis when you enter a function, Excel adds the parenthesis after you press Enter, as long as it’s obvious where the parenthesis is supposed to go. (Relying on this feature can produce unpredictable results; for accuracy, always double-check your parentheses.)