From someone who was an expat there for a period: It is the dialect you need to learn to get along in China. It is based on the Beijing dialect (but my friends told me it was not exactly the Beijing dialect), but it has been made the official language of China. When I was getting ready to go to China, my Chinese colleagues told me: Just learn Mandarin. Don't worry about the local dialects. If you can understand and be understood in Mandarin, you will be okay anywhere in China. I found their advice to be good advice, although I noticed a few differences in words in some of the places I went, and I noticed a big difference in accent between Beijing and Shanghai. What I heard spoken on the street in Shanghai often was not Mandarin, but when I spoke in Mandarin, I was always understood.

the Chinese language is the official language of China

Yep,I agree with AlexM's pefect answer(then I could type little~)China use Mandarin as official language,so we could hear Mandarin on TV,broadcasting station or see it on presswork,but we Do not say such "high-standardized Mandarin",so please don't feel astonished when you talk with your friends whose mandarin sounds quite different from what you heard before.(๑´ω`๑)

大家，谢谢 for all the information.

nearly 80% or more people in China can speak chinese-mandarin, so u are right. They CAN speak do not mean they speak chinese-mandarin in their daily life. People in different provices usually speak their own dialect with the natives unless u speak chinese-mandarin to them.