The difference between the two is where they are obtained from a plant.
Herbs come from the leafy and green part of the plant.
Spices are parts of the plant other than the leafy bit such as the root, stem, bulb, bark or seeds.
Examples of herbs include basil, oregano, thyme, rosemary, parsley and mint.
Herbs are obtained from the leaves of herbaceous (non-woody) plants.
Spices are obtained from roots, flowers, fruits, seeds or bark.
In some cases both a herb and spice may come from the same plant. Dill is an example of this.