Botanically speaking, all nuts are seeds, but not all seeds are nuts. Many dried seeds and fruits are called nuts, but true nuts are both seed and nut in one and cannot easily be separated.
Seeds are found in fruits and may be removed from them, as with almonds, cashews and pistachios. Botanists describe nuts as a simple dry fruit with one seed (sometimes two) in which the ovary wall becomes very hard (stony or woody), with the seed remaining unattached to that wall. True nuts include chestnuts, hazelnuts, pecans and walnuts. The other nuts widely used by bakers are described as culinary nuts or seed nuts. The distinction is only important in botany. Any large, oily kernel found within a shell and used in food can be considered a nut.