There are many (perhaps hundreds of) Mandarin dialects. As the official language of Mainland China, Taiwan, and Singapore, Mandarin has many regional variations which are usually referred to as Mandarin dialects.
What is a Mandarin Dialect?
Each region where Mandarin is spoken has developed its own version (or dialect) of the language. There can be variations in vocabulary, grammar, and pronunciation, but the language remains understandable to people from other regions. This is the feature that differentiates dialects from languages.
The standard Mandarin of Mainland China is based on the Beijing dialect, which is spoken in Beijing and the Beijing region. Almost every city in China has its own variation on standard Mandarin, and each of these could qualify as a Mandarin dialect.
Besides Mandarin, there are many other languages which are spoken throughout China and Taiwan. Cantonese, Hakka, Taiwanese, and Wu (which includes Shanghainese) are just some of the many languages spoken in the Chinese region.
These languages are not considered dialects because they are not mutually understandable. There may be some similarities due to the common root of these languages, but speakers of Taiwanese and those of Cantonese (for example) will not be able to easily communicate with each other.
The Common Language
Since China is such a large country with more than a billion people, having a common language allows for easier administration. Mandarin serves that purpose, and in fact is known as the “common language” (pǔ tōng huà).
As well as the spoken language, written Chinese also provides a linguistic common ground. The same Chinese characters are used for several different languages, including Mandarin, Cantonese, Hokkienese, and can even be used for basic communication in Japanese.
Standard Mandarin and the written Chinese language provide a common ground for the Chinese, and are used to facilitate communication between people from widely separated regions.