An herbarium is a collection of pressed and dried plant specimens.  Typically the specimens are arranged according to some scheme of classification and are readily available for study. The goals of an herbarium are (1) to serve as a reference collection for checking the identity of newly obtained specimens, (2) to aide in the teaching of
systematic botany, (3) to provide an historical record of the flora, and (4) to provide a body of data for biological research, especially a survey of biological diversity.

Making herbariums was a popular pastime for Victorian gardeners, particularly due to the popularity of 'plant hunting' all over the world at that time. Of course, many of these have not survived because of the difficulty in storing them, and the quality of the materials used. If you are interested in making an herbarium of your own, follow the usual methods of gathering leaves, flowers and even a whole plant with roots. Dry either in the microwave or silica gel, or even the old-fashioned way by pressing them in heavy books.

Make sure that the materials you use are archival, museum-quality, acid-free papers.  Label each specimen as completely as possible:  botanical name, common name, date and place the specimen was picked, and any other information you want to add, including uses for the plant.

An herbarium is a wonderful and unusual gift for a gardener, too.