The letter ו represents a consonant phoneme that, depending on the form of Hebrew, is pronounced either [v] (like the <v> in the English word vine) or [w] (like the <w> in the English word wine). Most modern-day forms of Hebrew, including Modern Israeli Hebrew, use the [v] pronunciation; but some, such as the traditional pronunciation of Yemenite Jewish communities, use [w], and it's commonly believed that Classical Hebrew used [w] as well. Additionally, Modern Israeli Hebrew has acquired a separate /w/ phoneme in various loanwords and proper nouns (mostly from Arabic and English); this phoneme is commonly written וו or ו.
The letter ו is also used as a vowel letter (mater lectionis) representing the vowels /o/ and /u/. In writing with vowel markings, the former is written as וֹ, the latter as וּ. However, neither /o/ nor /u/ is always indicated with a letter, especially in traditional spelling. (In Modern Israeli spelling, they are often indicated even where traditional spelling did not indicate them.)
In Modern Israeli spelling, ו as a consonant is sometimes doubled within certain words, even when pronounced [v], to avoid confusion with ו as a vowel.
For information about ו as a conjunction meaning roughly “and”, see ו־.
For information about ו as an ending meaning roughly “his” or “him”, which is one form of the third-person masculine singular personal pronoun, see ־וֹ.
For information about ו as an ending in third-person plural past and future tense forms, see Appendix:Hebrew verbs.