Dieresis
Spanish words that contain -gue or -gui are pronounced by omitting the u sound and making a hard g.
For example:
guerra, guerrilla, albergue, Águeda...war, guerrilla, youth hostel, Águeda (female name)...
guiso, águila, siguiente, alguien...stew, eagle, next, someone...
But there are a few words in Spanish containing -gue or -gui that are pronounced differently: the sound of the u is not omitted and the g is still hard. To indicate this, the written word uses the orthographic symbol called a "diéresis". This symbol looks like two dots above the letter u. Some examples using the dieresis are:
cigüeñastork
lingüísticalinguistics
vergüenzashame
pingüinopenguin
bilingüebilingual
Notice how the pronunciation varies if we do not include the dieresis:
pinguino (incorrect)penguin
pingüinopenguin
U is the only Spanish letter that can have a dieresis