A topic which commonly raises its head in Chinese learning forums is whether or not Cantonese, or indeed other languages within China, are languages or in fact dialects. There is a nice article by Robert Lindsay (not the British sitcom actor) that outlines a dialect requires 90% familiarity with the mother language. My personal view will always be that Cantonese is a language.
If you are interested in the subject the following two articles are well worth a read, especially the comments at the bottom of the second one:
There are also some interesting maps showing the variety and spread of languages across China, Hong Kong & Macau, Taiwan, and occupied Tibet (original source). No surprise is that the main boundaries are formed by the former countries which are now collectively referred to as China: