The name itself indicates its likely origin, which was use in religious mass. One of the earliest references to the vino was found in Florentine wine merchants' logs in the renaissance era, as the wine was marketed to Rome and environs specifically for religious ceremonies. Another theory of the origin of its name states that the tradition of fermenting the wine starting around All Saint's Day and bottling it around Easter lent the wine its "holy" appeal.
Eventually this style of wine was produced on the island of Santorini in Greece when it came under Ottoman rule. Their biggest audience was the east: Greek vin santo was widely exported to Russia and became the standard wine used in Russian Orthodox mass.
Like other dessert wines, vin santo gets its sweetness from the dessication, or drying process, as mentioned earlier. This concentration of the sugars in the grapes creates its signature sweetness. Sometimes a madre, or mother (starter) is used, taking a bit of previously-produced vin santo and pouring it, with all its wild yeasts, into the new bottling to jump-start the fermentation process.
This is thought to give the wine complexity and a bit of continuity in character from harvest to harvest. The flavor profile of vin santo is heavy on nutty and/or raisin notes, with some cream and honey in the mix as well. Most wine producers in Tuscany tend to produce Chianti for the masses, but they squirrel away a barrel or two of vin santo to enjoy in famiglia (with the family)...Proof again that Italians know a thing or two about keeping the good stuff among loved ones.