If you are referring to the sound of it, "w" is a semi-vowel, which means it's also a semi-consonant. It is also called a rounded high back glide.
If you mean the symbol "w", it seems to me that you could think of it as the vowel sound /u/ in words like 'few' /fyu/. It is frequently the second element of a vowel digraph (aw, ew, ow), but I'm not sure if that's what you had in mind.