The IPA phonetic symbol [w] represents a voiced labio-velar approximant, a semivowel present in most accents of English in words such as week or queen and corresponding to IPA phoneme /w/.
This sound is also present at the end of IPA phoneme /uː/, particularly before a vowel, i.e. /uː/ as [ʉw] or [uw]. For example continuous /kənˈtɪnjuːəs/ pronunced as [kənˈtɪnjuwəs].
Some accents also have IPA phoneme /əʊ/ pronounced as [əw] or [ow], for example rower /ˈrəʊər/ as [ˈrowɚ].
IPA phoneme /aʊ/ can be pronounced [aw] as in shower /ˈʃaʊər/ as [ˈʃawɚ].
Notes and references