vowels and consonants are sounds, not letters. letters are more of suggestions for sounds that should be made when spoken.
for instance, "yet" and "wet" feature semivowels y and w; whereas the w in "cwm" is a vowel, as it makes a vowel sound
-- Edited by MATHSEX at 00:57, 2009-02-16
Right. The letters that represent semi-vowels, usually called a "glide" in English, is a left-over from various runic alphabets. In Latin, and in some dialects of English, W and Y were never used because people recognized that those sounds are created through a diphthong. Those letters represent vowel sounds; they cannot make a consonant sound.