American people speak English, same as British people. Sure, there's a different accent, some say "truck" and the others say "lory", but still it's all just English. So when talking about Chinese, why do we have to mention whether it's Mandarin or Cantonese?
Well, unlike the different kinds of English, Mandarin and Cantonese are totally different and unintelligible to one another. Yes, they are both written using the same writing system (almost, look at the previous post), but so is English and French.
Mandarin and Cantonese speakers each have a different cultural heritage. Most of mainland China speak Mandarin. The Beijing way of pronouncing words in Mandarin is considered the standard, they call it "Putonghua". When you listen to the news in China, that's the language that they speak.
If you ask most Chinese people: "What language do you speak?" they will answer: "Putonghua".
Cantonese is spoken in the region formerly known as Canton in South China, today's Guangdong province, mainly in the big cities of Guangzhou and Hong Kong. While fewer people speak this kind of Chinese ("only" several tens of millions), they take much pride of their language being closer to the classical, old Chinese - unlike the northern Putonghua that was greatly influenced by the many nomadic conquerors along Chinese history.
Hong Kong's native language is Cantonese, though foreign people mostly think its simply "Chinese"
Therefore, when doing business with Chinese people, make sure you know what kind of Chinese they are speaking! Sometimes you will find out that your basic assumptions are wrong... Each kind of Chinese (more will be introduced in following posts) carries a unique cultural charge - understand it, and you will gain valuable credit with your counterparts.
There are more kinds of Chinese! They are spoken by several minorities in and outside of China.