what is the difference between the word "Chinese" and "Mandarin"?

i am confused.

Chinese refers to all people from China.  Mandarin is a language spoken in China, and Cantonese is also a language spoken in china.  There are many different dialects of each, but these are the two main languages in China.

There is no language actually just called "Chinese" 

you see,most  chinese people speak one language, and we have different dialects, such as cantonese used in the southern part of china, which is so different that people from north can't even understand it.

Ah, I see.  Well, in english, Chinese refers to your nationality (like I am Canadian, you are chinese) and Cantonese refers to your language (I am English and you are Cantonese) So, although you may be both, they do not mean the same thing. 

China has long history with rich culture, it is natural that many foreigner even some chinese don't know clearly some facts. China has at least hundreds of dialects, for example, in my hometown in southeast china, when going to a neighbor village on the other side of a mountain i can not understand what they said. Here I list serval major dialects in china.
1. Beijing dialect
    we can think that mandarin just is the Beijing diaglect. the "man" in the word "mandarin" means the "man nation" in northeast china. Beijing diaglect bacome the china official language about 400 years ago, 250 years before the making of Canada. TaiWan has been using the same official language. Further earlier the china official language is another diaglect.
2. Guangdong dialect( Canton is the old name)
    The Canton has no comprehensive paper language like mandarin, it is mainly used in Guangdong province and HongKong. Because HongKong is more known to foreigner compared to other cities in china, so foreigner know about  Canton.
3. Xiamen diaglect( Amoy is the old name)
    Xiamen diaglect has no paper language, it is used in Fujian province and TaiWan.
4. Hakka dialect
    Hakka diaglect is used in Guangdong, Fujian, HongKong, Taiwan.
    The 3  dialects above are the major china language used in Southeast Asia and China town in Europe and US and Canada.
5. Tibet diaglect
    Tibet diaglect is a nother language with another culture. Actually the history of china is also a war history.
6. .....

That's all very interesting, Aforest, but I don't think the question was about the entire history of dialects of China…..

I think you have gotten the correct answer, quote youself as saying that Chinese refers to all people from China.  Mandarin is a language spoken in China.

Chinese also refers to the written language used in China which, I believe, is the same whatever dialect you speak.