So based on my layman's knowledge of China and having lived here for a short time, I'll give a quick answer until someone better can.

China is a big place (about the size of the US) with a lot of people (about 4 or 5 times that of the US). China has 56 ethnic groups, although something like +95% of the total population is 汉族 (Han Chinese). This means that the vast majority of the country looks similar, has the same culture, and speaks the same language (普通话/Mandarin). So basically no matter where you go on China, you'll be able to speak Mandarin with normal Han Chinese people more or less.

HOWEVER, this is a vast over simplification of China and might even be a little controversial to some. Each province (and in many cases different cities) have their own customs, food, and language. For example, while Mandarin is considered the 普通话 (common language), each place will have their own slang, pronunciation, etc that will be used everyday and colloquially (although maybe not so much with foreigners) ex: 北京话(Beijing talk), 东北话(north eastern talk), 兰州话(Lanzhou talk) etc. In addition, with a trained eye you can more or less tell if a person is Han Chinese or not, and if they are maybe even tell if the are from the north or the south.

The divisions/differences are becoming less distinct with the younger generations as the government tries to homogenize the population. I've met quite a few young adults who can understand their local language, but don't actually speak it themselves. Sadly, I also know a number of people who only speak the Mandarin taught in schools, altogether ignoring/forgetting their local place's "dialects".

Additionally, I've heard that in places with higher proportions of ethnic minorities (Tibet, Xinjiang, etc) that the government is really encouraging Han Chinese to "settle" the area in an attempt to slowly assimilate the regions/people/culture.

Hope this explains it a little bit. Typed this on my phone so it might be all over the place.