I think I got it now.
They are words unlike vowels for which the air is stopped.
A vowel on the other hand can be a word where the air is not stopped by your lips or tongue.
(Quote from .wisegeek.com)
What is a Vowel?
are sounds for which there are one or more points where air is stopped. In nearly all languages, words must contain at least one vowel. While a word can be formed without any consonants – such as the English words I or way – no word may consist of only consonants, without a vowel.