The classic Sugar Skull tattoo is one with rich cultural history behind it. They are based on the decorative skulls made for the Mexican holiday Dias De Los Muertos or Day of the Dead. Day of the Dead is starts on November 1st and ends the next day. Its purpose is to honor those close to one who have died. The decorative skulls are usually made from sugar, hence the name sugar skull, and are placed on an altar.
The sugar skull tattoos are usually just as decorative as the skulls they are based on. They usually have designs inside the lines of the skull such as flowers, crosses spades, shapes, and hearts. Sometimes, they also have decorations on the outside such as flowers, usually marigolds or roses, a candle on the top of the head, hats, or veils. Some are just a skull, some are a normal face with sugar skull designs on it, and others have skeleton bodies. Some sugar skulls just have white backgrounds while others have colors.
Sugar skulls made for day of the dead are said to bring good luck. Many people carry around small ones for good luck. I have a feeling, sugar skulls being lucky is why people started getting them as tattoos. Either way they make for beautiful tattoos that can be integrated with a lot of other tattoo elements such as flowers, hearts, and leaves.