Good luck charms, fetishes, amulets, talismans, mojo bags, and similar items and images generally exist as apotropaic symbols, with highly variable forms and meanings, originating in the local mythologies and superstitions of a religion, place, or culture.
Some good luck charms are carried on one's own person as jewelry or hidden in pockets; some are painted onto boats or stitched into clothing; some are tattooed into the skin or woven with thread into the hair. A good luck charm may be an extremely personal symbol, only significant to one person, such as the manual gear shift knob of a car in which you survived a head-on collision. Others might be heavily entrenched in religion, such as a crucifix necklace, a Hamsa, or a Nazar. Still others might be culturally detached from their origin story: a gambler may carry a rabbit's foot on him, but he may not necessarily know why the rabbit has any connection to luck.
Depending on the culture of origin, the specific "function" of a good luck charm may also vary considerably: it may exist specifically to ward off bad luck or negative spiritual influences, as is much the case with amulets in Haitian Vodou; it may exist to attract the favour of "Lady Luck," a deity, or another personification of safety or a favourable outcome. It may even exist specifically to inflict harm and ill fortune on the carrier's enemies or competitive opponents.
Good luck charms have gained increased popularity as jewelry items with little direct association to luck: charm bracelets may have many small trinkets or symbols attached to them, purely for aesthetic purposes and personal expression of the wearer's interests.
Good luck charms also occur as actions and verbal incantations, not just as objects: a blessing from a religious figure may be considered a good luck charm; so might pouring water or wine onto the ground for luck, or knocking on wood. In countries where the Evil Eye is widely accepted, if you compliment a woman's child, she may immediately spit on the ground or insult her child loudly, to thwart the potential negative attention the child could receive from spirits. Hockey and other sports players, gamblers, and pickpockets have been widely known to adopt an immense variety of superstitious rituals, such as wearing one sock inside-out, or repeatedly wearing the same piece of clothing unwashed during a success streak, in order to avoid souring or disrupting the success they have enjoyed.