UHT stands for Ultra High Temperature. It is a non-chemical heat process that extends the shelf-life of Milk up to 55 days. The Milk is heated to about 280 F (137 C) for a minimum of 2 seconds to kill all traces of bacteria in the Milk.
The process is normally applied only to cream, which people use more slowly than milk, and therefore requires a longer storage life.
Some people swear that they can detect a taste difference, and don’t like it.