Dill weed refers to the fresh or dried leaves of the dill plant (Anethum graveolens), as distinct from dill seed, the plant's dried seed, which has a stronger, more caraway-like flavor. "Dill weed" is most often used on spice labels to specify the dried leaf form, sold as small green flakes, though the term is sometimes used loosely for fresh dill fronds as well.
Dill weed has a grassy, slightly sweet flavor with a subtle licorice-like note. It's a staple in Scandinavian and Eastern European cooking - flavoring fish, potatoes, pickles, and creamy sauces - and also appears in Indian and Middle Eastern dishes, added to curries, rice, and yogurt-based condiments. It pairs especially well with salmon, cucumber, sour cream, and lemon, as in gravlax, tzatziki, and potato salad. Because dried dill weed loses potency over time, it's best added toward the end of cooking and replaced every few months.