The letter u ultimately comes from the Semitic letter Waw by way of the letter y. See the letter y for details.
During the late Middle Ages, two forms of 'v' developed, which were both used for its ancestor 'u' and modern 'v'. The pointed form 'v' was written at the beginning of a word, while a rounded form 'u' was used in the middle or end, regardless of sound. So whereas 'valor' and 'excuse' appeared as in modern printing, 'have' and 'upon' were printed 'haue' and 'vpon'. The first distinction between the letters 'u' and 'v' is recorded in a Gothic alphabet from 1386, where 'v' preceded 'u'. Printers eschewed capital 'U' into the 17th century and the distinction between the two letters was not fully accepted by the French Academy until 1762.
Use in English
In English the letter is pronounced differently in different words. As an English vowel letter, it has a "long" and "short" pronunciation that are used in different contexts. Short 'u', found in closed syllables, commonly represents /ʌ/ (as in 'duck') or /ʊ/ (as in 'put'). Long 'u' commonly represents /juː/ ("long U" as in 'mule') or /uː/ (as in 'blue'). In a select few words, short 'u' may also represent /ɪ/ (as in 'business').
Additionally, the letter 'u' is used in text messaging and internet and other written slang to denote 'you', by virtue of both being pronounced /juː/.