By default, a cell lies in a given row and column. Sometime, however, you may want a cell to "drop down" to the next cell below it, or perhaps stretch across into the next row. This is accomplished using the COLSPAN and ROWSPAN attributes, respectively.
Recall that, when you create a TR row, you must make sure that the number or cells in the columns in the row sum to the correct number (determined by the very first row). In doing so, you must take into account the cells that span across multiple columns (COLSPAN) or hang down from a previous row (ROWSPAN).