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.