Let me answer your question just generally, not grammatically, because I am not from Canton or Hong Kong. 1. CANTONESE Cantonese is just one dialect in China. Some Americans also asked me such a question like you did. After I thought it over, I thought and still think that is probably because of the famous international city "Hong Kong" or many Cantonese who lived abroad during the 20th century or the famous Kongfu star "Bruce Lee" or the isolated time of China before 1978. Foreigners especially after the foundation of PRC learnt Chinese mainly through Hong Kong and Cantonese who lived abroad. So, Cantonese is known to almost every foreigner although it is just one kind of dialects in China. 2. MANDARIN As for Mandarin, it is like standard English, e.g. BBC English. China is so big that almost every place has its own dialect, some similar to Mandarin such as many dialects in north China and Sichuanese, others quite different from Mandarin such as Cantonese and Shanghainese. It was promoted after the foundation of PRC, maybe in 1956 as playboy said and became popular during my generation. I was born in 1982. I remember that at the time when I went to school (1988), it was a great pride to say good Mandarin. Generally we don't use Mandarin when we're at our hometowns. For instance, I just used and still using Sichuanese in Sichuan (one province in southwest China) like other Sichuanese except when I am outside of Sichuan or speak to guys from other provinces. --------------------------------------------------------------------------------------------------------------------------- ALMOST every Chinese can Mandarin, BUT MOST Chinese CANNOT Cantonese.

the differrences are 1) sounds （ can say totally different) 2) grammar ( a little bit similar ) 3)tones ( Mandarin have 4 tones , and Cantonese have 9 tones) 4) writing( yeah, they use the same charrecters but the meaning doesn't fit all the time(itz more like using chinese charrecter to write cantonese words by sounds,(the same way as most of languages share Latin alphabet). BUT now the modern cantonese tend to be more import the same way as madarin in both of grammatical & the writing, and this is new thing.) for example the same sentence in madarin but the cantonese will keep exactly in madarin but read it as in cantonese sounds ( to share the same sentence in mandarin.) this is the result of the modern cantonese get a big influent from mandarin. and because of the historical & political involved that central china gorverned toward canton regions many centuries . esp since 17 centuries, the mandarin became popular by the central gorvement to force all the regions to use mandarin as official language instead their own languages in order to communicated with all others ethnic groups throughout the Mainland(china). so since than the cantonese obsurbed and consumed madarin lotz through schools, media and so on.

They are like, two totally different languages. Tho they both are Chinese.

Mandarin have 4 tones , and Cantonese have 9 tones

the tones :P