Autosum is a much widely used feature in Excel. It is available in two places in Excel 2013. One in Home tab and one in Formulas tab.
You can quickly access the AutoSum by the shortcut key ALT+
Now, for starters what is AutoSum also needs to be talked about. When you click AutoSum it enters a Sum formula by guessing the cells vertically (above the cells and left horizontally where AutoSum has been pressed or ALT+ has been taken. It doesn't work vertically down and horizontally right)
It also takes into account only continuous range of cells. If it encounters a discontinuity, it will take into account only that much range which is continuous.
AutoSum becomes more powerful if required cells are selected and then AutoSum is chosen (or ALT+ is pressed). This removes all the problems of vertically down, horizontally right and discontinuity problems as shown above. Though still the answer would appear only down in case of vertical selection and right in case of horizontal selection.