Herbarium specimens are dried and pressed plants stuck on a sheet of cartridge or other archival quality paper with a label attached in the bottom right hand corner to indicate provenance, collector, number and identity. Additional information, such as local uses, is also often included in the label information. Kew's Herbarium collection comprises herbarium specimens cross referenced with ancillary collections, such as the carpological collection, mostly for fruits and items too big to fit on a sheet of paper, and a spirit collection, used to store fragile items that would loose their tridimensional shape once pressed, especially orchid flowers.
In the Herbarium, specimens are arranged systematically in the cupboards by family, region, genus and species, so that anyone can find an example of a particular species within minutes. It is, as it were, a card index box of the world's plants, all 400,000 plus species, in a single building but with the sheets arranged systematically to reflect affinities and, usually, evolutionary relationships, rather than alphabetically.
More details on the preparation of herbarium specimens and their curation can be found in Bridson & Forman, 1999, The Herbarium Handbook, available from www.kewbooks.com.