Maraschino is a liquor made from the Marasca cherry, a sour cherry from Dalmatia. The cherries are picked when ripe, crushed with pit intact, mixed with honey, and allowed to ferment. The resulting cherry wine is distilled into a clear liquor that tastes of cherries and has a hint of almonds. In ages past, the maraschino cherry was preserved in this liquor.
Maraschino liquor is exported to the United States by the Luxardo company. It is an essential component in several mixed drinks of earlier eras. However, in this day and age, it is a far cry from "the Italian liquor best known and most appreciated all over the world" as the Luxardo label optimistically states. When mixing a drink that calls for "maraschino", quell your reflexive reach for the jar of cherries. Rather, assess the drink with a careful eye: if it contains gin, the drink wants the real thing and there will be no substitution; if it has a vodka base, perhaps it actually wants the pink ichor.