It's easy to come up with a table design that requires a cell that spans several rows. An example is something where the left-most column labels the rest of the table; this can be done (in simple cases) by using diagonal separation in corner cells, but that technique rather strictly limits what can be used as the content of the cell.
The multirow package enables you to construct such multi-row cells, in a very simple manner. For the simplest possible use, one might write:
\begin{tabular}{|c|c|}
\hline
\multirow{4}*{Common g text}
& Column g2a\\
& Column g2b \\
& Column g2c \\
& Column g2d \\
\hline
\end{tabular}
and multirow will position "Common g text" at the vertical
centre of the space defined by the other rows. Note that the rows
that don't contain the "multi-row" specification must have empty
cells where the multi-row is going to appear.
The "*" may be replaced by a column width specification. In this
case, the argument may contain forced line-breaks:
\begin{tabular}{|c|c|}
\hline
\multirow{4}{25mm}{Common\\g text}
& Column g2a\\
& Column g2b \\
& Column g2c \\
& Column g2d \\
\hline
\end{tabular}
A similar effect (with the possibility of a little more
sophistication) may be achieved by putting a smaller table that lines
up the text into a *-declared \multirow.
The \multirow command may also used to write labels vertically
down one or other side of a table (with the help of the
graphics or graphicx package, which provide the
\rotatebox command):
\begin{tabular}{|l|l|}
\hline
\multirow{4}*{\rotatebox{90}{hi there}}
& Column g2a\\
& Column g2b \\
& Column g2c \\
& Column g2d \\
\hline
\end{tabular}
(which gives text going upwards; use angle -90 for text going
downwards, of course).
Multirow is set up to interact with the bigstrut
package (which is also discussed in the answer to
spacing lines in tables). You use an
optional argument to the \multirow command to say how many of the
rows in the multi-row have been opened up with \bigstrut.
The documentation of both multirow and bigstrut is to be found, as comments, in the package files themselves.
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=multirow