Functions are the nuts and bolts of Excel. There are a few ways to enter a function into a cell.

#1)
Entering a function directly into a cell or into the formula bar:

First:

Here we want a function that returns the average quantity of apples on hand.

Next:

Excel filters the functions available as you start typing.

Next:

Complete the entered function by supplying the required arguments. In this case, the range of cells for which we want an average returned.

The Result:

The average quantity of oranges on hand at all warehouses.
#2)
Entering a function using the Insert Function button:
Note:

If you already know the function you want, it's much easier to just enter it directly. However, if you are unsure which function to use, or even if there is a function that does what you want, this method is best.

You can search for functions in this dialog or scroll through the functions within a specific category (Statistical Functions in this example).

Note:

The Function Arguments dialog can be very helpful when entering complicated functions with several required and optional arguments.

The AVERAGE function in our example is a relatively straightforward function.

The same result as above.

Summary

Functions are the nuts and bolts of Excel and can be entered directly or with the help of the Insert Function dialog. It generally depends on how familiar you are with desired function.