"Chinese" is really just an overall term for a language(s) spoken in China. It is not a language.
There are hundreds of languages in China.
There are two more well known one:
Mandarin - This is spoken all over pretty much all over China, and the Chinese translation for this means 'standard language'. Generally if someone says they can speak 'Chinese', this is what they are referring too.
Cantonese - This is the language spoken in Hong Kong and the area of the South East (Guangdong). It's has lots of similarities to Mandarin, and a Mandarin speaker may be able to understand a bit, but won't be able to speak it themselves.
Each city and village will also have it's own dialect and even language. Now though, most people will speak and understand Mandarin. The smaller villages and minority groups may only know a very small amount, and the older generations may know none at all.
I live in China