Functions

Excel has several hundred built-in functions that make Excel more powerful and life easier for us users. The most important categories of functions are mathematical functions, text functions, and lookup and reference functions. SUM(), AVERAGE(), COUNT() are some examples of functions.

Functions are used in formulas. Some examples of how functions can be written are:
=SUM(1,2)
=SUM(A1,A2)
=SUM(A1:A100)
=ROUND(SUM(A1:100),0)

Before you can start writing a function in a cell, you need to begin with an equals sign (=) to show that you intend to write a formula and not a value.

You can then type the function using the keyboard or use the Insert Function tool  found under the Formulas tab in the ribbon.