The two predominant types of Sherry are Fino (very dry with a lighter-body) and Oloroso (still dry, but much richer in both flavor and body). If the winemaker is going for Fino, alcohol is added (fortification) until it reaches just over 15%; however, if Oloroso is the goal then alcohol is added to reach an 18% alcohol content.
Now the fun begins, while the wines remain in their casks they are permitted contact with air in the top portion of the cask. A layer of yeast, called "flor" forms a coating on the surface of the Sherry, keeping the wine from over oxidizing - these wines will become Finos as their lower alcohol content is what allows the yeast to grow in the first place. Olorosos on the other hand do not support the growth of flor due to their higher alcohol content. Olorosos are permitted to oxidize intentionally, producing a darker, and richer wine, with more body than a Fino.
Sherry wines must go through a solera system for adequate aging. This system is essentially a blending system of casks that hold wines of varying ages. The oldest casks are the ones that are bottled in a given year and the next casks are arranged in such a way that the youngest Sherries are blended into a series of casks holding progressively older Sherries. The blending off of younger Sherry into older Sherry results in very consistent, high quality wines that all share a portion of the oldest, original vintage of Sherry made at the bodega. Sherries do not have a vintage date, per se, as they are really a blend of many years.
Producers to try include: Osborne, Emilio Lustau, Hidalgo, Gonzalez Byass