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What is an Herbarium?
The Herbarium as a Collection of Preserved Plants
The Herbarium as a Research Institution
The Herbarium as an Educational Institution
The term herbarium, used in its strictest sense, refers to a collection of preserved plant specimens. An herbarium is much like a library 
that contains plants rather than books. The collections in the herbarium, like those in the library, represent a vast wealth of information that is invaluable for teaching and research. Thus, in the broader and modern sense, an herbarium is not just a collection of dried plants, but it is also an educational and research institution.
The Herbarium as a Collection of Preserved Plants
The collections of most herbaria include seed plants (angiosperms and gymnosperms), ferns and their allies, and bryophytes (mosses, liverworts). Many herbaria also include fungi, lichens and algae. In addition, an herbarium may specialize in particular groups of plants (such as palms) or plants from a specific geographic region. Herbaria range in size from small personal collections to massive ones such as at the New York Botanical Garden with more than 5.3 million specimens.
Just as a library is much more than a collection of books, so too is an herbarium more than a simple collection of plants. Many herbaria have collections of wood, fruits/seeds, botanical art, taxonomic literature and other materials. As an aside, the term 'herbarium' didn't always refer to collections of plants. Originally, the term was used to describe books about medicinal plants. However, by the mid-eighteenth century, during the reign of the great Swedish taxonomist, Carl Linnaeus, the term had acquired its present meaning.
The process of pressing and drying plants for storage has been amazingly successful. For example, specimens prepared by Linnaeus in the 1700's still look much as they did in his day. Properly prepared and maintained, herbarium specimens should last indefinitely. Thus, not only are herbarium specimens scientifically valuable, they are important historical records that can span many generations. Specimens record the works of famous botanists from all over the world.
Herbarium collections are typically arranged by plant family in a manner that reflects evolutionary ancestry (phylogeny). One arrangement used in many herbaria is that of Engler-Prantl with modifications by Dalle-Harms. Although this system doesn't reflect recent trends in taxonomic thought, many herbaria, especially larger ones, continue using it because it would be physically impossible to switch. Smaller and/or newer herbaria may be organized by more recent phylogenetic systems such as that proposed by Arthur Cronquist (1993). Some herbaria are even arranged alphabetically by family. Although this arrangement makes it easier for a novice to locate specimens, it suffers from the disadvantage that if the name of plant family changes (which is not uncommon), then the entire herbarium would have to be rearranged. A phylogenetic system would need to only change the label.
The Herbarium as a Research Institution
Since each specimen is accompanied by an assortment of information (i.e., scientific name of the plant, where and when it was collected and by whom), an herbarium represents a vast warehouse of raw data. Herbaria document: (a) the vegetation of an area; (b) the taxon (group) to which a specimen belongs; (c) the appearance of a plant in a particular locality at a particular time of year; (d) the range of variation that exists within a species; (e) the nature of evolutionary processes and the evolutionary relationships among plants; (f) the phenology (life cycle in relation to season such as when it flowers or ripens fruit) of a plant; and (g) vegetative changes that occur at a site over time.
Herbarium specimens also provide materials that can be studied away from the field or during another season (i.e., winter). Often, specimens are deposited and stored in a herbarium as vouchers. These specimens serve as a reference, or as a proof of the identity of a plant that was used in a particular scientific study. One particularly important voucher is the "Type" specimen - which is the specimen upon which a plant name is based.
Yet another important function of herbarium specimens is that they can be used as a reference to check the tentative identification of an unknown plant. Unknown plants can be compared with herbarium specimens that have already been identified.
In short, an herbarium is an invaluable scientific repository for information about plants. Herbarium specimens are often used in studies that were probably never even dreamed of at the time the collection made. Since the specimens in the herbarium represent an irreplaceable (and hence priceless) source of data, access to these collections is usually closely monitored, as is the access to rare books in the library, to prevent damage.
Although herbarium specimens can be considered priceless (since they are irreplaceable) or nearly worthless ("if you've seen one redwood you've seen 'em all"), the currently accepted value by the American Systematics Collections is approximately $10/specimen. This figure includes the value of the collector’s time, the curator's time, electricity and other costs inherent in storing the collection. Thus, a herbarium represents a major institutional resource.
The Herbarium as an Educational Institution
A major role of any herbarium is that of education. Most herbaria have collections specifically for use by students. These collections are typically kept separate to prevent unnecessary damage to the permanent collections.
The teaching collections, in conjunction with the permanent reference collections, provide materials that are essential for teaching courses such as Plant Taxonomy, Plant Ecology, Plant Kingdom, and Plants and Human Affairs. Learning characteristics of plants, studying relationships between plants, and identifying unknown plants are just a few of the many educational uses for herbarium specimens.
Just like a museum or library that offers educational programs and other services to the public, so does the herbarium. For example, an herbarium may host lectures, organize field trips, prepare botanical displays, provide tours, identify plants among other activities. In many instances, the herbarium serves as the focal point for botanical activities in an institution.