Most people regard "Chinese" as being a single language, as did I while growing up in Canada. My first interest in learning Chinese began about 10 years ago as a result of my having many Chinese associates. I was very curious about the language they used to speak to each other, so I picked up my first Chinese textbook and tried to practice what I was learning with them. I soon found out that what I was studying and what they were speaking were two different languages. That was when I learned that the Chinese you hear out West can usually be classified into either Cantonese or Mandarin. At the time, the vast majority of what "Chinese speakers" spoke outside of China was Cantonese. This confused me as I now had to decide whether to learn the official language of China or learn what people on the street were actually speaking.
Fast forward to a few years ago when I first landed in Taiwan and began to seriously learn Chinese. Surely, living in a Chinese speaking country and constantly hearing what I was studying would greatly speed up the learning the process? I made a new discovery. While people could understand what I was saying and I could understand them when they were speaking to me, I couldn't understand them when they were speaking to each other! (Deja vu!) After more research, I learned that what most people in China learn and speak at home isn't Mandarin! Every province or region has its own "dialect" that people use to speak to each other, that are as different as English and French or Italian and German. In total there are about 50 distinct dialects across China and overseas Chinese communities, not counting regional variations between them.
This leads to an interesting situation for foreigners learning Chinese and wanting to communicate with Chinese people. A great analogy I heard is to imagine yourself walking into a party dressed up while everyone else is dressed casually. You might hear "Wow, you look great!" or "Nice outfit!" and while it feels great to be complimented, you can't help feeling left out for being the only "non casual person."
The prospects aren't gloomy though. All media is broadcast in Mandarin as that is the official form. Similarly, for business transactions or formal occasions, you can expect Mandarin to be used. It is also the language used when traveling or when strangers meet, so it is definitely the language to learn. The only time local dialects are used is when locals speak with other locals from the same region. For most learners, I would recommend keeping your focus on the big picture (Mandarin), although you can win yourself some points by learning a few key phrases in the local dialect to show respect.
