1. Dust or earth taken from a grave. Some people suggest that it should only be from the grave of a person who would approve of its use. If you want to use it as part of a luck spell, collect it around the new moon. For warfare, pick it up around the full moon. Collect it at midnight and leave an offering such as money, food, or liquor. It should be something that the deceased person liked.
2. Powder ground up from the herb patchouli or leaves from mullein. This can be used alone or with other herbs to magickally bring some sort of restriction or inertia.
3. Although a popular and literally-interpreted term in some folk magic books, graveyard dust is actually a code used in spells and recipes by Pagans, ancient doctors, alchemists, etc. going back to the Greek Magical Papyri. It means Mullein.