Chinese is made up of many dialects that exemplify the cultural diversity of the country. In fact, some have argued that there really is more than one language contained under the umbrella of Chinese. Even the Chinese people distinguish the written word from the spoken word, recognizing that the two are not the same.
While there is almost an unlimited number of Chinese dialects that make up the country, generally scholars recognize anywhere from seven to eleven of them as the major dialects of the country.
The Chinese dialects are as different from one another as Cockney is to English. For the spoken word, it's near impossible for a person of one dialect to understand someone in another. It's even arguable that within large dialect groups there can be enough variance to be hard for people to understand one another. This later example is comparable to English spoken in England vs. America. Thus the divisions within a dialect are often referred to base on where they originate, such as the "Bejing Dialect of Mandarin" or simply just "Bejing Dialect".
While the spoken words are different, the written word is the same. Thus comes some of the confusion in the West when trying to Anglicize, or put it into the English language, the Chinese words. For instance, the capital of China has been spelled both "Peking" and "Bejing". The city itself didn't go through a name change. The two pronunciations of the capital are different because of the different dominant dialects that ruled at the time.
Today, the most widely spoken dialect of Chinese is Mandarin. This was the language used by the people who ruled China through the last dynasty, the Manchurian. Within the Mandarin dialect, there are a variety of variations that change the sound of the words to being "softer" or "harder", very similar to how people in Boston drop their "r"s and people in Texas tend to roll their words more.
The major dialects are:
The major differences in the dialects are in how the intone the words and the sounds. For instance, Mandarin has four major tones, whereas Yue has up to eight.
Tones are much like accents. We use it without thinking in English because in English, the tones don't change the meaning of the word.
So in English, we might have a directive with the word "you":
You are a good person.
And a question:
How are you?
Without changing the meaning of the word.
In the dialects of Chinese, that questioning tone at the end of the second statement could very well change the meaning of the word. In addition, there are over 4000 Chinese written words and not nearly as many different sounds we as humans can articulate. Thus, in Chinese, it becomes very important to not only understand how the word is spoken but also the context it is used in.
