The voiced labio-velar approximant is a type of consonantal sound, used in certain spokenlanguages, including English. It is the sound denoted by the letter ⟨w⟩ in the English alphabet; likewise, the symbol in the International Phonetic Alphabet that represents this sound is ⟨w⟩, and the equivalent X-SAMPA symbol is w. In most languages it is a labialized velar approximant[ɰʷ], and the semivocalic counterpart of the close back rounded vowel[u] - i.e. the non-syllabic close back rounded vowel. As labio-velar consonants do not easily fit into consonant charts with only labial and velar columns, [w] may be put in either the velar column, (bi)labial column, or both, though the latter is rare outside of the official IPA chart; the placement may have more to do with phonological criteria than phonetic ones.
Features of the voiced labialized velar approximant:
Its manner of articulation is approximant, which means it is produced by narrowing the vocal tract at the place of articulation, but not enough to produce a turbulent airstream. The type of approximant is glide or semivowel. The term glide emphasizes the characteristic of movement (or 'glide') of /w/ from the /u/ vowel position to a following vowel position. The term semivowel emphasizes that, although the sound is vocalic in nature, it is not 'syllabic' (it does not form the nucleus of a syllable).
Its phonation is voiced, which means the vocal cords vibrate during the articulation. However, in some languages, such as Swiss German, it can just mean that this consonant is pronounced shorter and weaker than its voiceless counterpart, while its voicedness or lack thereof is not relevant. In such cases it's more accurate to call such sounds lenis or lax.
It is an oral consonant, which means air is allowed to escape through the mouth only.
It is a central consonant, which means it is produced by directing the airstream along the center of the tongue, rather than to the sides.
Martínez-Celdrán, Eugenio; Fernández-Planas, Ana Ma.; Carrera-Sabaté, Josefina (2003), Castilian Spanish, Journal of the International Phonetic Association33 (2): 255–259, doi:10.1017/S0025100303001373