The Spanish letter U has some unique characteristics.
1. U is normally pronounced "oo."
2. In terms of syllable division, U is a weak vowel:
3. In terms of consonant pronunciation, U is a
|mucho gusto||my pleasure|
(a) Between G and a soft vowel, U is normally silent.
|guía telefónica||phone book|
Note: if U is to be pronounced, it needs a dieresis.
(b) When a C or G needs to be pronounced "hard" in front of a soft vowel, the letter U plays a role – see hard and soft vowels
Please note that this explanation is only a guideline for the Spanish that I know, which is Castilian Spanish. There are many regional variations in Spanish pronunciation.