A discussion from Thomas Henick-Kling

FORTIFIED WINES: PORT, SHERRY, MADEIRA
fortified wine (18-20% alcohol): addition of grape spirit (wine distillate) stops fermentation, preserves sweetness

LIQUOR MUSCAT
fortified Muscat juice left to concentrate in barrels at high ambient temperature

SHERRY
Spanish Sherry: grapes: Palomino and Pedro Ximenez
Yet any grape variety can be (and is) used. The Sherry making process obscures grape varietal characteristics. The flavors produced by the oxidative handling and the Sherry yeast are dominant.

Flor Sherry
Grapes harvested at 18-20 ¡Brix, pH 3.0-3.4. Initially fortified to approx. 15% alcohol. Then filled into barrels which are left with large air space (ullage). Flor yeast is added. This yeast (a type of Saccharomyces cerevisiae) form a thick mat on the surface, producing the characteristic fermentation and oxidized flavors (acetaldehyde 150-300 mg/L). Later the wine is transferred to other oak casks (filled completely) and left for further maturation.

Other Methods:
submerged, aerated culture; column Flor growth

PORT
A large variety of grapes are used (including some white grapes). The high sugar content (26-40 Brix) of the grapes typically used for Port production requires a osmotolerant and alcohol tolerant yeast to start fermentation. Grape spirit added at 14-18 ¡Brix, fermentation slowly stops, residual sugar in Australian Port wines 7-12 Brix, Portuguese Ports: 6-8 Brix, 17-19% alcohol.

Vintage and Tawny Port styles.
Vintage Port is fruitier, darker color, more tannic, less oak aged, all one vintage.
Tawny Port is lighter in color, longer oak aged, blend of different years to maintain a consistent style.