Approximant /w//w/ is a voiced labial velar approximant. It's the sound that most Brits who speak General British produce at the beginning of this sentence: "Woe gates here zoo den Laser roymen." (sorry for the weak joke)
Seen aerodynamically /w/ displays laminar airflow, so - in auditory terms - there's no audible friction. If I'm not mistaken, the term 'approximant' was first proposed by the late Peter Ladefoged in 1964 in his monograph A Phonetic Study of West African Languages, in which he writes on p. 24 (I quote from the second ed. of 1968):
"The term approximant is used here to describe a sound which belongs to the phonetic class vocoid or central resonant oral [...], and simultaneously to the phonological class consonant in that it occurs in the same phontactic patterns as stops, fricatives and nasals."
[w] is an approximant; others would call it a frictionless continuant. We need both lip-rounding (at least in careful speech) and a frictionless narrowing of tongue and velum to articulate the sound. That's why the sound is classified as labial velar. It's called 'voiced' because the vocal folds vibrate.
Q: What do you call snakes on your car windscreen?
A: Windscreen vipers.
1. The wurst is yet to come.
2. Wurst is best.