When you enter a long bit of text into a cell, it will spill over into the next cell (provided the next cell is empty). Sometimes, this can be exactly what you want. If, however, the text you are entering is a column heading, the spill over can make it confusing.
In the Format Cells dialog (Format>Cells), there is an option to Wrap Text. You’ll find it on the Alignment tab.
With Wrap Text checked, Excel will find a natural place to break the text and increase the row height to boot. An example of what this might look like is below.
As you can see, just because Excel finds a natural split doesn’t make it right. In the above example, it would look nicer if “(in thousands)” was all on one line. You can force a line break by using ALT+ENTER (hold down the Alt key while pressing Enter) when your cursor is in the position where you want the break. By placing the cursor just before the opening paren and pressing ALT+ENTER, the following result can be achieved