Usually, in linguistics, /y/ is considered a semi-vowel or semi-consonant (which is quite the same). This means that it neither is a full consonant nor a full vowel. It's a mix of both of them. Anyhow, in some languages the letter "y" may be, as already noded, used as a full vowel.

The reason why /y/ is regarded a semi-consonant lies in its phonological definition as a "palatal approximant". This means that, when producing the sound [j], your tongue approximates your palate, but does not touch it (if it would touch it, it would create the german "ich"-sound, [ç] in the IPA).

Another semi-consonant is the english /w/ (also [w] in IPA), which is a bilabial approximant.