Excel's AutoFill feature is activated by dragging the little square that appears in the lower right corner of the active cell. Here are the options for copying with AutoFill:
If the cell contains a formula, dragging the AutoFill box will copy the formula to all cells over which you drag. The formula will be updated to logically respond to the information in the cells to which you copy. In other words, using AutoFill to copy the formula =A1+A2 one cell to the right would result in the formula =B1+B2.
If the cell contains text, dragging the AutoFill box will copy the text to the cells over which you drag. If the text is contained in a custom list (such as the days of the week or the months of the year), dragging the AutoFill box will result in filling the adjacent cells with entries from the list.
If the cell contains a number, dragging the AutoFill box will copy the number fo all cells over which you drag.
If the cell contains a number, holding down the CTRL key while dragging the AutoFill box will increment the number by one in each of the cells over which you drag.
To change the increment in the previous example, enter properly incremented numbers in two cells, then select both cells before dragging the AutoFill box to new cells. For example, if I enter 1 in cell A1 and 3 in cell A2, select cells A1 and A2, and drag the AutoFill box down through cells A3, A4, and A5, those cells will be filled with the numbers 5, 7, and 9.
If you double-click on the AutoFill box of an active and occupied cell, the cell contents will be copied vertically if there is an obvious vertical location. For example, if cells A1 through A5 are filled with numbers, and B1 through B5 are filled with numbers, and I enter this formula in cell C1:
and then double-click the AutoFill button in cell C1, the formula will be copied to cells C2 through C5 (and the copied formula will adjust itself as it goes through the rows, so in cell C2, the formula will read =A2+B2, and so on)