Difference between Spices and Herbs

A simple answer to this question would be that the leaf of a plant which is used in cooking is referred to as a Herb and any other part of the plant that is used to flavor food is termed as a Spice.
For example - Cloves (bud), ginger & turmeric (roots), fennel, cumin, coriander (seeds), cinnamon (bark), peppercorns (berries).

Herbs are used fresh and spices are generally in dried form.