Mandarin and Cantonese share characters and a name - "Chinese" - and are mutually unintelligible when spoken. A Mandarin speaker and a Cantonese speaker cannot converse in their native languages; they can write notes to each other.
How different they are
- Tones: Mandarin has 4 + neutral; Cantonese has 6-9 depending on how you count entering tones. Cantonese conserves the final consonants (-p -t -k -m) Mandarin lost: 十 is sap6 vs shí.
- Vocabulary and grammar: close but distinct - 系 vs 是， 唔 vs 不， 佢 vs 他. Everyday Cantonese sentences frequently cannot be transcribed with standard characters.
- Writing: formal written Cantonese converges to standard written Chinese (based on Mandarin) - so the two read the same newspaper but speak it differently.
Where each is spoken
Mandarin: mainland China, Taiwan, Singapore's standard, ~900M+ native speakers. Cantonese: Guangdong (esp. Guangzhou), Hong Kong, Macau, and much of the historic overseas diaspora, ~85M speakers. Everywhere else in the Sinosphere, Mandarin is the lingua franca of school and state.
Which to learn
For virtually every learner: Mandarin first. It is the standard everywhere, has 10x the learning material, and its phonology is kinder. Learn Cantonese for Hong Kong life, family heritage, or Hong Kong cinema - and know going in that the tone system is harder and the resources thinner. The dialect field guide maps where Cantonese sits among the other Sinitic languages.
Common questions
Are Cantonese and Mandarin the same language?
Linguistically they are separate languages - mutually unintelligible in speech. They are "dialects" politically and share standard written Chinese, which is why the confusion persists.
Is Cantonese harder than Mandarin?
Yes for learners: 6-9 tones vs 4, more final consonants to distinguish, far less learning material, and a written/spoken split where textbooks differ from speech.
Should I learn Cantonese or Mandarin?
Mandarin - it is the standard in mainland China, Taiwan and Singapore with far more resources. Cantonese for Hong Kong/Macau life or heritage specifically.