no difference :)

http://en.wikipedia.org/wiki/Cantonese

Chinese divided into two parts: Cantonese and Mandarin. But China has 65 ethnic groups, each place has a different dialect.Cantonese is widely used in these places: Guangdong, Hong Kong and Macau. there is no diffrence.

basically they have almost the same pronouncation，but their biggest difference is like the difference between British English and American English.

I think the main difference is that cantonese in Guangdong somehow like to use wordings similar to that appear in Chinese (or say, more formal Cantonese) while Cantonese in Hong Kong may be more informal in comparison. For example, 鼠标(GD) VS 滑鼠(HK), 空調(GD) VS 冷氣(HK) But generally, whichever wordings you use can be understood in both places.