According to The Herb Society of America's New Encyclopedia of Herbs and Their Uses by Deni Bown:
"The term "herb" also has more than one definition. Botanists describe an herb as a small, seed bearing plant with fleshy, rather than woody, parts (from which we get the term "herbaceous"). In this book, the term refers to a far wider range of plants. In addition to herbaceous perennials, herbs include trees, shrubs, annuals, vines, and more primitive plants, such as ferns, mosses, algae, lichens, and fungi. They [herbs] are valued for their flavor, fragrance, medicinal and healthful qualities, economic and industrial uses, pesticidal properties, and coloring materials (dyes)."