Mandarin" is the Chinese dialect spoken by senior officials in Beijing (also called "mandarin" by foreigners in Marco Polo's time). But this dialect was not only common among the officials - in fact it is the dialect of North China from the area around Beijing.
During the Cultural Revolution, Mandarin was officially designated as the national language to create a standard on which every citizen in China can communicate with other citizens.
Today, this High Chinese is spoken in all public institutions throughout China, schools teach in High Chinese and the national television program is broadcast in High Chinese.
Mandarin is the Chinese you should learn if you want to be able to communicate in China and/or Taiwan.