Herbs have a variety of uses including culinary, medicinal, or in some cases even spiritual usage.

The green, leafy part of the plant is often used, but herbal medicine makes use of the roots, flowers, seeds, root bark, inner bark (cambium), berries and sometimes the pericarp or other portions. General usage differs between culinary herbs and medicinal herbs. A medicinal herb may be a shrub or other woody plant, whereas a culinary herb is a non-woody plant, typically using the leaves.

By contrast, spices are the seeds, berries, bark, root, fruit, or other parts of the plant, even leaves in some cases; although any of these, as well as any edible fruits or vegetables, may be considered "herbs" in medicinal or spiritual use. Culinary herbs are distinguished from vegetables in that they are used in small amounts and provide flavor (similar to spices) rather than substance to food.

In this sense, some herbs contain phytochemicals that when consumed in small quantities can be healthy, yet in large quantities can be toxic to the liver. Some types of herbal extract, such as the extract of Hypericum perforatum (St. John's wort), or the Piper methysticum (kava plant)can be used for medical purposes to relieve depression and stress. However, administering high amounts of these herbs may lead to poisoning, and should be used with caution.