Spices are the buds, bark, roots, berries and aromatic seeds that are harvested for use in flavouring cooking. Herbs are the leaves of plants, so when we use coriander leaf we refer to it as a herb, however when we use coriander seed we say we are using a spice
It is true that most herbs are leaves, whereas spices are seeds etc, but an exception to this is the Bay Leaf which is usually referred to as a spice, rather than as a herb. Another broad distinction is that generally, spices come from tropical plants and are associated with more highly flavoured oriental dishes, whereas herbs grow in temperate climes and are used in milder European food.