When you click Excel's AutoSum button from a cell next to a row or column of numbers, Excel automatically sums those numbers.
You don't have to type a formula (=sum()) or drag through the cells. Excel figures out what's needed and adds the precise summing you need.
Now here's the really nice part. There's a keyboard shortcut that lets you do this without having to change your focus from the worksheet to the toolbar.
In Windows, just press the Alt key and = (the equal sign).
A related tip is that Excel automatically sums all selected cells, ALL THE TIME. Select a bunch of numbers, and check out your status bar at the bottom of the window.