Generally, spices come from any part of a plant that is not the leaves and herbs are only the leafy part of the plant. Herbs can be fresh or dried and spices are most often dried.
Plants that are considered herbs include thyme, rosemary, parsley, bay leaves and cilantro. Spices include peppercorns, ginger, nutmeg and saffron. Although they are all parts of a plant, they are not the leaves, as herbs are. Peppercorns are berries, ginger is a root, nutmeg is seeds, and saffron is the stamen from a flower.
Although the terms "herbs" and "spices" are used interchangeably in many cookbooks, there is a difference between the two. Herbs typically grow in temperate regions of the world because they tend to be more delicate than spices. Spices can include everything from bark to fruit and grow in a variety of different climates, all over the world. Another major difference between the two is that herbs are more fragrant and flavorful when they're fresh. Spices tend to give off more flavor when they've been dried.
Herbs are generally defined as being flowering plants "whose stems above the ground do not become woody." Spices are "any class of pungent substance of vegetable origin used as a seasoning or preservative."