"Chinese" is a language family wearing one name. Linguists count seven to ten major groups - mutually unintelligible languages that happen to share a writing system and a state.
The major groups
- Mandarin 官话 - the north and southwest, ~900M; standard Mandarin （普通话） is its cultivated form. This is "Chinese" to the world.
- Wu 吴 - Shanghai, Suzhou, Hangzhou; ~80M. The language of classical Shanghai culture.
- Yue 粤 - Cantonese and relatives; Guangdong, Hong Kong, ~85M.
- Min 闽 - Fujian and Taiwan's Hokkien/Teochew; ~75M worldwide with the diaspora.
- Xiang 湘， Gan 赣， Hakka 客家 - Hunan, Jiangxi, and the scattered Hakka communities respectively.
"Dialect" is the wrong word
Calls them 方言 (fangyan, "regional speech") - but Shanghainese and Cantonese differ from Mandarin as much as French from Spanish. The political unity is why the word "dialect" survives: one state, one written standard. Within Mandarin itself, "dialect" is more honest - Sichuanese and Beijing speech are real dialects, mostly intelligible.
What it means for a learner
Learn standard Mandarin - it is the lingua franca: a Shanghainese taxi driver speaks Wu at home and Mandarin to you. The regional languages are for the places you settle or family heritage. The dialect field guide walks each group with audio where it survives in the corpus.
Common questions
How many Chinese dialects are there?
Seven to ten major groups (Mandarin, Wu, Yue/Cantonese, Min, Xiang, Gan, Hakka, plus Jin/Huizhou/Pinghua by some counts) - really distinct languages; hundreds of local varieties below that.
Can Mandarin speakers understand Cantonese or Shanghainese?
No - mutual intelligibility between the groups is essentially zero. They share written Chinese and a name, which is why they are called dialects.
Is Taiwanese a dialect of Mandarin?
No - Taiwanese Hokkien is a Min-language, unrelated to Mandarin beyond the shared script. Taiwan's Mandarin (国语) is a Mandarin variety; the two coexist.