Cantonese is a dialect, with very different sound system, although it shares the same writing system with Mandarin. The difference between their sound systems could be larger than that between English and German, or between Russian and and Bulgarian. It makes sense if one considers the size of China. It is as big as Europe sans Russian. There are as many dialects in China as languages in Europe. In some remote parts in East China, the people in the towns next to each other could not understand each other's dialects. So a common writing system was enforced more than 2000 years ago so that people could communicate with each other in writing.
Native Cantonese speakers is less than 5% of total population in China. http://en.wikipedia.org/wiki/Chinese_language. It is mostly spoken in Canton and Hongkong. A helpful analogy (not accurate definitely) is to think of it as Spanish in the scope of Europe. A native Mandarin speaker will not understand Cantonese if he doesn't study it, just as a native English speaker will not understand Spanish without study.