Ailanthus, as Inyo writes, is a tree that was introduced to North America from Asia and is sometimes considered to be a pest.
The Ailanthus is most notable for its long (over 1 foot) leaves, which are pinnately compound, and have between 12 and 18 leaflets. When it is young (and it usually is), it has a smooth, almost green bark. When it gets older, its bark looks more like a normal tree, although it still seems strangely flexible.
Ailanthus can also be told by two others characteristics: when it blooms, it stinks; and it can grow almost anywhere. I have seen Ailanthus grow up to four feet tall in two inche cracks in pavement within a year.
As Inyo noted, it can be a very invasive plant, although to be fair, it usually grows in areas that have already been disturbed, such as near rail road tracks, abandoned lots, and also riparian areas. But since the Ailanthus has only been in America for a 100 years, we do not yet know whether it permanently colonizes these areas, or whether it will follow a normal route of succession.
The name of the plant derives from the Indonesian name, I believe, modified with anthus, the Greek for "flower".