The Semi-consonant /w/
The features of /w/ are: very high, back, rounded and non-syllabic. It must precede a vowel and is never found at the end of a word. Examples of words containing /w/ are ouest [wɛst] , jouer [ʒwe] , oiseau [wazo] .
Distribution of /w/
French avoids the sequence of "high vowel + vowel". That's where semi-consonants come in. Rather than "high vowel + vowel", French prefers the sequence "semi-consonant + vowel. Consider the word jouer. The letters "ouer" suggest that the transcription would be [ʒue]. However, the correct transcription is in fact [ʒwe] where the semi-consonant /w/ is used rather than the high vowel /u/. This, of course has implications for the number of syllables. There is only one in the word jouer.
Articulation of /w/
While /w/ is similar in English and French, keep in mind that French /w/ is shorter and tenser. When articulating, the tongue is higher than for /u/, almost to the point of causing friction.
Spelling of /w/
The main spelling representation of /w/ is "oi"/"oy", e.g.: moi [mwa], voyage [vwajaʒ] or "ou" + vowel, e.g.: jouer = [ʒwe]. There are two good examples in the Canadian word for bullfrog:
ouaouaron [wawaʁõ] .
There is one notable exception to the idea that "ou +vowel" = "/w/ + vowel". It involves cases where the "ou + vowel" sequence is preceded by a consonant followed by /l/ or /ʁ/. In the latter case, use /u/ rather than /w/ e.g.: clouer = [klue] . The root of this pronunciation seems to be ease of articulation since three consonantal elements in a row is quite a mouthful!
Note that the letters "OUILLE" represent the sequence [uj], e.g.: mouiller [muje] , nouille [nuj] , citrouille [sitʁuj] .
The actual letter "W" is rare and French (and is really only used in borrowings). It can sometimes represent the sound /w/, e.g.: wifi [wifi], but can also represent /v/, e.g.: wagon [vagõ].
• never use /w/ at the end of a word
• the sequence /u/ + vowel is only found after clusters involving a consonant followed by "L" or "R"
•"ou" + vowel usually represents /w/ + vowel
•"ouille" represents [uj]