Fortified wines were originally produced in order to help preserve wine during shipping.  The process of creating a fortified wine is based on the addition of spirits, most commonly grape brandy, before the fermentation process is complete to produce fortified wines.  This addition, whether grape brandy or a neutral spirit, can add specific flavors to fortified wines, and the alcohol added will stop the yeast production process and result in fortified wines that are sweeter and stronger.  Some of the most popular fortified wines are Madeira, Marsala, Port, and Sherry.  Fortified wines such as Vermouth is added to traditional martinis.