Semivowels are vowels that function as consonants. They are typically briefer, less stable and often closer than the corresponding vowels.
Palatal semivowels correspond to front vowels, velar semivowels to back vowels, and labialized semivowels to rounded vowels.
Examples:
English y in yes (SAMPA [j]), a palatal or unrounded front semivowel.
English w in well (SAMPA [w]), a velar labialized or rounded back semivowel.
French hu in huit (SAMPA [H]), a palatal labialized or rounded front semivowel.