Before you can work with a cell or range, you must select it, and when you do, it becomes active. The reference of the active cell appears in the Name box at the left end of the formula bar. Only one cell can be active at a time, but you can select ranges of cells. You can change the active cell in a selected range without changing your range selection.
Selecting with the Mouse
To select a range of cells, drag the mouse over the range. Alternatively, instead of dragging through all the cells you want to select, you can use a technique known as extending to indicate two diagonal corners of the range. Simply click the cell at one corner of the range you want to select, then hold Shift and click the cell at the corner of the range diagonal ...