Cantonese (or Yue) is one of the five major Chinese languages. These are often called "dialects", but in actuality their differences are great enough to consider them separate languages.
Cantonese is spoken by about 100 million people in the southern provinces of Guangdong and Guangxi and in neighboring areas such as Hong Kong and Macao, as well as throughout South-East Asia in such places as Singapore, Malaysia, Thailand and Vietnam. Due to the migration of Cantonese speakers from Hong Kong and the Guangdong area, Cantonese is the dominant form of Chinese spoken in the Chinatowns of many major cities in the United States, Canada, Australia and elsewhere.
The word Cantonese comes from Canton, the former English name of Guangzhou, capital of Guangdong, which was once considered the home of the purest form of Cantonese. However, through years of mass media and pop culture influence, Hong Kong has now truly become the cultural centre of Cantonese.
Although Mandarin (or putonghua) is the standard and official language in mainland China, it has only been around for about 700 or 800 years, compared to the 2000-year history of Cantonese. Cantonese, not Mandarin, is the dominant language in overseas Chinese communities. This comes from the fact that, around the world, the largest flow of Chinese immigrants originates from Hong Kong.
Cantonese is mainly an oral language. People in Hong Kong use standard Chinese (putonghua) when they read and write. They speak Cantonese in their daily interactions with people. As a colloquial language, Cantonese is full of slang and non-standard usage. The language of youth is rapidly evolving, and new slang and trendy expressions are constantly emerging.
The standard written language in Hong Kong is essentially the same Chinese as everywhere else in China. The only difference is that Hong Kong and overseas communities, like Taiwan, have kept what are called traditional characters, whereas mainland China uses simplified characters. In an attempt to increase litteracy in China, thousands of characters were "simplified" in a 1950 spelling reform initiated by chairman Mao Zedong.