The shortcut is
Alt + Enter
To insert a line break within a cell hold down the Alt key (next to your space bar) and press Enter. This inserts a fixed line break within the cell.
This operates differently to the Wrap Text format. Wrap Text will break depending on the length of the words and the width of the column.
The Alt + Enter method will break in the spot you added Alt + Enter providing the column is wide enough. This method is ideal for data lists that require a single heading row. Instead of using two rows for the headings (not a good practice) you can use one row and get the same effect as two rows.