Excel allows you to quickly and easily fill a series of cells with a progression of values by simply using the fill handle. This is the small black dot that appears at the lower-right corner of a cell when it is selected. Just click on the fill handle and drag to select the cells you want filled. Using this feature allows Excel to make decisions about how the cells you select should be filled.
You can gain more control over the filling process if, instead of clicking on the fill handle, you right-click on it and then drag. When you release the mouse button a Context menu appears that allows you to select the type of fill you want to perform.