Ü or ü is a letter not used in English. It started as an U with an E above it. It is heavily used in the Turk languages, such as Turkish. In German, the letter can be replaced by Ue or ue respectively, if it is not available on the keyboard.
Germany and Austria
Ü or ü is one of the 4 extra letters used in German. It can be replaced by using the letters Ue or ue. In English language newspapers it is often written as U or u but this is not correct.
Internet addresses are written as "ue" because the internet address system can only understand ordinary English letters.
German is one of the official languages of Switzerland, people from Switzerland who speak German normally use the extra letter.