What is Chinese language? Is it just one language? Is Chinese equal to Mandarin? No! Actually Chinese is a group of related, but in many cases very distinct, language varieties, also known as dialects.
Mandarin, which is called Putonghua as well, is a standardized form of spoken Chinese based on the Beijing dialect. It is the official language of Mainland China.
The most well known dialect is undoubtedly Cantonese, which is commonly known as Guangdonghua. It is spoken within Guangdong Province, Hong Kong, Macau and some areas in southeastern China. It is the official language in Hong Kong. Mandarin is by far the most widely spoken language, with up to 960 million speakers in China, while Cantonese is a relatively small language, with 80 million native speakers.
There are seven main regional groups of Chinese, which are mandarin, Yue (known as Cantonese), Min, Wu, Xiang, Hakka and Gan, covering more than 200 individual dialects. These classifications are based on geographical location in China.
Besides the main dialects, there are some other languages used by various ethnic groups. China is made up of 56 ethinc groups with over 90% of the population belonging to the majority group, Han. Many of the minority groups have their own language, such as Tibetan, Muslim chinese population (Hui), Mongolian, etc. However everyone learns Mandarin when they go to school since Mandarin is the official language all over China.
Most Chinese can understand Mandarin but not everyone can speak Mandarin well. For instance, most people in Guangzhou speak Mandarin with very strong Cantonese accent, which might be pretty hard to understand for foreigners. Generally northern dialects are much easier to understand than southern dialects. So If you plan on doing business in China or traveling around, Mandarin is the main language to learn.