Technically, a tea is any herb, leaf or other plant substance steeped in hot water. Tea (with a capital letter) is an aromatic beverage commonly prepared by pouring hot or boiling water over cured leaves of the Camellia sinensis, an evergreen shrub (bush) native to Asia. Tea is but one term for a liquid brewed from the leaves of these plants. Chai is just another asian word for tea. All cultures have had teas made from all manner of herbs. The word tea is associated with the trade in Camellia sinensis and the singular asian word adopted to reference it. There are actually many.
My personal favorite type of tea is Earl Grey which is a black tea flavored with oil of bergamot.