What is the difference between an oo and a yoo vowel sound?

The long OO sound is the sound of a long U, as seen in Flute and dune (floot, doon). This is an exception to the concept that long vowels "say their name." The YOO form may be considered just a variation of the long OO, or as a separate vowel sound.

The long YOO (and a possible short YOO) sounds like the letter U, either because it is beginning a Word (unique, uniform) or because certain vowels come before the long U. You can hear the YOO in cute, mute, mule, feud, fuel, pupil, and pure. Most dictionaries will use a consonant Y before the OO or long U to indicate this pronunciation.

Some dictionaries avoid the distinction by using an umlaut U instead of a long U to indicate OO.

* The reason many vowel lists avoid pairing the OO and YOO sounds is that saying "long U" can be confusing where there are both OO and YOO versions of Words: cute/coot, mute/moot, feud/food, fuel/fool.