Milk tea is commonly consumed in England, Ireland, Scotland, Canada, India, Sri Lanka and Hong Kong. There are many variations on milk tea, including a cream tea prepared with heavy cream (found in East Friesland, Germany), a type of tea prepared with evaporated milk (known as Hong Kong Milk Tea or Pantyhose Milk Tea), the tea latte (found in North America, parts of Europe and beyond), and milk tea that is boiled with cow milk, buffalo milk or yak milk (found in many parts of India and surrounding countries).
Milk tea is usually prepared with a strong black tea. The addition of milk results in a less astringent, less bitter flavor, in part because the milk covers up the flavor of the tea and in part because the milk binds to some of the bitter chemical compounds found in tea. (Some of these bitter compounds are very beneficial to the health, which is why, if you're drinking tea for health reasons, tea without milk is a better choice.)
For more information, see this list of milk tea recipes from around the world, including drinks from the UK, India, Malaysia, Thailand, Japan, Taiwan, Morocco and elsewhere.