Eucalyptus is one of three similar genera that are commonly referred to as "eucalypts", the others being Corymbia and Angophora. Many species, but far from all, are known as gum trees because they exude copious sap from any break in the bark. Eucalypts have many local names, like 'gum trees', 'mallee', 'box', 'ironbark', 'stringybark' and 'ash'.
Nearly all eucalypts are evergreen but some tropical species lose their leaves at the end of the dry season. The leaves are covered with oil glands. These oils are often used to treat coughs and colds.
Many eucalypts also change the shape of their leaves as they get older. Young eucalypts have round leaves. When one to a few years old, the leaves of most kinds become longer and spearhead or sickle shaped. A few kinds keep the round leaf shape all their lives. Most species do not flower until the adult leaves starts to appear.
Eucalypts have many uses which have made them important. Because of their fast growth, the main benefit of these trees is the wood. They provide many uses, such as planting in parks and gardens, timber, firewood and pulpwood. Fast growth also makes eucalypts suitable as windbreaks. Eucalypts draw a very large amount of water from the soil. They have been planted (or re-planted) in some places to lower the water table and reduce the amount of salt in the soil.
Eucalyptus oil is refined from the leaves by steaming. It can be used for cleaning, deodorising, and in very small amounts in food supplements, especially sweets, cough drops and decongestants. Eucalyptus oil is an insect repellent