The Rowan is a tree or sometimes shrub in the Rose Family. Rowan can be used to refer to either the specific species Sorbus Aucuparius or to any tree in the Sorbus family at all.
Whatever the technicalities, the Rowan tree has some things in common: they have pinnately compound leaves, and bunches of bright red berries. They also tend to grow in cold climates and\or high altitudes. Like other members of the Rose family, the seem to range in size between large shrubs and small trees.
They usually resemble other members of the Rose Family in such matters as the pentangle shape on the berries (which is perhaps the source of the rumors about their magical powers), the bisexuality of the flowers, the small amount of seeds produced, and of course, the cynanogens in the seeds. Despite the cyanide and the popular misconception, the berries are not notable poisonous, and they taste to bitter that it would be hard to eat enough to harm yourself.
The Rowan is found in temperate North America quite commonly, as a decorative plant. However, it's natural range is confined to the more notherly parts of the United States and into Canada, especially at higher altitudes. In the mountains of Montana, I have actually seen varieties of Sorbus growing almost as thickets.