Common Name: 

Five species of coniferous evergreen trees, of small to medium size, and a few shrubs, usually dense.   Bark is scaly.   Branches usually horizontal or also ascending, developing a conical crown.   Twigs more or less flattened.  Leaves opposite, scale-like, appressed, lateral leaves partly overlap facial leaves.  Cones small, ovate or oblong.  Native to North America or eastern Asia.    Many cultivars of different size, form and color are available.   The foliage of several types discolor in winter, reducing their acceptability.  Some tend to thin out with age and attain an unkempt appearance.  Arborvitae are especially popular in the Midwest and eastern U.S.
Note: There is general consensus that one species, Thuja orientalis, does not belong in this genus and has been transferred to the genus Platycladus, hence Platycladus orientalis.   However, most horticultural nurseries and retail outlets continue to use the older designation.
Thuja: the Greek name of juniper, or from thyia, a reference to a scented gum, possibly that from Thuja articulata.