Ivy is a n evergreen climbing vine in the genus Hedera. A number of vines within this genus are colloquially referred to as "ivies," and some plants outside this genus are sometimes confusingly called "ivy" as well, perhaps most notably poison ivy. Ivy has been cultivated for centuries as an ornamental plant, and some people find it quite attractive, although others have raised concerns about ivy, citing that it can become quite invasive, especially in areas where the plant lacks natural biological controls.
Several traits are common to the ivies, allowing people to easily identify them. These vines have very woody stems, periodically marked with adhering discs which put out roots, allowing ivy to grip its substrate. The plants also have palmately lobed leaves, which may be solid or variegated, depending on species and cultivar, and they produce small umbels of green flowers which mature into berries in the fall and winter.
As an ornamental, ivy certainly has some advantages. It is very fast-growing, for one thing, and its evergreen nature ensures that it will act as a cover year-round. This is especially convenient when ivy is used to conceal ugly walls, as the plant can be coaxed into growing quickly, essentially causing the wall to disappear. Ivy is also extremely hardy, which can be an advantage in marginal climates, and it can act as an air scrubber, filtering toxins out of the air, which is very useful in urban areas.
However, ivy does have downsides. Outside of the Old World, where ivy is a native, the plant can compete with native species, and it frequently chokes out more slow-growing, delicate native plants. In North America especially, ivy has been known to smother trees, causing them to die and eventually fall in storms. Ivy can also be hard on structures, as it tends to pull buildings apart with its extensive vine and root system.
If you do not live in an area where ivy is native, you may want to think carefully before planting ivy. Native plant societies may be able to provide you with a list of alternatives which can be used like ivy in landscaping and design. If you have an existing ivy plant, there's no need to rip it out, but you may want to keep an eye on it, and control its growth where possible. In the case of a building overwhelmed by ivy, contact an architect, who can determine whether or not the ivy is damaging the building and take steps accordingly.