A simple answer is people in China.
When we say “speak Chinese”, “Chinese” means Standard Chinese, Mandarin, Putonghua (普通话). Besides Mandarin, people speak some dialects in different parts of China. For example, most people living in Guangdong(Canton) province speaks Cantonese. But not all people living in Canton speaks Cantonese. Teochew and Taishanese are also important dialects in Canton. You may hear them in Chinatown all over the world. Don't panic if you have learnt Chinese for a long time and find that you can't understand what local people are saying.
People living in Hong Kong and Macao speaks Cantonese and don’t know much about Mandarin because they were ruled by U.K. and Portugal for more than one century and have few communication to north of China whose native language is Mandarin. But things are changing after they return to main land China.
In 1949, National Party lost its power to Communist Party and retrieved to Taiwan. So Taiwan’s resident are from all over China and speak Mandarin (may have slight accent difference).
Here is a map of the Sinophone world.