Researchers working in the herbarium at Appalachian State University
An herbarium is a place that houses plant specimens. Many people expect that an herbarium will be filled with living plants. However, these plant specimens are pressed, dried and glued to sheets of paper. They are dead. The sheets of paper are typically stored inside of cabinets for safekeeping and organizational purposes. An herbarium is similar to a library in that both house artifacts (specimens or books) in a safe and organized way that allows them to be found and used by researchers. If plant specimens are stored properly they will last hundreds of years with little sign of degradation.
If you think about it, it makes sense to store plant specimens in this way (as dead samples). Logistically you can fit and care for many more pressed specimens than living plants in the collection. In addition, an herbarium often contains plants from all over the world including plants from diverse habitats such as deserts and alpine tundra. It would be tough to grow all of these different plants in one place.
Herbaria are sometimes part of natural history museums, associated with botanic gardens or academic institutions. The largest herbarium collections in the world are housed in Paris and New York. These large collections contain millions of specimens. However most herbarium collections are much smaller. There are more than 3500 herbaria scattered across the world. In fact, you likely have a local herbarium that contains specimens of the plants that occur in your area. To learn more about the herbaria in your area you can search Index Herbariorum, a global directory of herbaria.