Wu is a language† spoken in East China, most popularly as Shanghainese. It is classified under the Sino-Tibetan language family and is closely related to many of the other languages spoken in China. It is not, by any means, mutually intelligible with Mandarin, the official language of China, though many cognates exist given their shared history.

In addition to differences in grammar and vocabulary, there are a number of phonetic differences as well. Wu contains a number of voiced initials whereas Mandarin, at least officially, has none. The voiced initial consonants tend to be only slack voiced, giving the Wu dialects their characteristic softness. There are also a number of pure vowels not found in mandarin. And, in most dialects of Wu, there are as many as 8 tones, twice that found in Mandarin.

You can find out more at Wikipedia or Google Books.

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† The jury, at least for some, is still out as to whether Wu qualifies as a language or as a dialect. I personally would not be easily convinced that it’s not presently a separate language. In truth it’s all shades of grey up to a point, which is why at one time French and Occitan and Catalan and Spanish were all just dialects of vulgar Latin. Except now they’re not. And so neither are Wu, Mandarin and Cantonese. That, at least, is the position this site will be taking.