The main difference is pronunciation, but there are also substantial differences in vocabulary, and some differences in grammar. Note also that Wu is a language group, rather than a specific dialect. The Wu group includes many languages, for example Shanghainese, Suzhou dialect, Ningbo dialect, Hangzhou dialect, Shaoxing dialect and so on, which are to some extend mutually intelligible, but as one ventures further south, the differences become large, with Wenzhou dialect being completely incomprehensible to speakers of other Wu languages. Compared with Mandarin, the tone structure of Wu dialects is much more complicated. The range of sounds seems to be wider too, though doesn't have retroflex sounds like zh, ch, sh in Mandarin.

Wu is a separate dialect/language of Mandarin Chinese

Soft suchou dialect language吴侬软语

Mandarin is a misleading word for the official language of China while wu is a local speech