Mandarin is the most common language in the world as it is the official language of Mainland China, Taiwan, and one of the official languages of Singapore. Thus, Mandarin is commonly referred to as "Chinese."
But in fact, it is just one of many Chinese languages. China is an old and vast country geographically speaking, and the many mountain ranges, rivers, and deserts create natural regional borders. Over time, each region has developed its own spoken language. Depending on the region, Chinese people also speak Wu, Hunanese, Jiangxinese, Hakka, Yue (including Cantonese-Taishanese), Ping, Shaojiang, Min, and many other languages. Even in one province, there can be multiple languages spoken. For example, in Fujian province, you can hear Min, Fuzhounese, and Mandarin being spoken, each being very distinct from the other.
Dialect vs. Language
Classifying these Chinese languages as dialects or languages is a contested topic. They are often classified as dialects, but they have their own vocabulary and grammar systems. These different rules make them mutually unintelligible. A Cantonese speaker and a Min speaker will not be able to communicate with each other. Similarly, a Hakka speaker will not be able to understand Hunanese, and so on. Given these major differences, they could be designated as languages.
On the other hand, they all share a common writing system (Chinese characters). Even though characters can be pronounced in completely different ways depending on what language/dialect one speaks, the written language is understandable across all regions. This supports the argument that they are dialects of the official Chinese language - Mandarin.
Different Types of Mandarin
It is interesting to note, though, that Mandarin itself is broken up into dialects spoken mostly in China's northern regions. Many large and established cities, like Baoding, Beijing Dalian, Shenyang, and Tianjin, have their own particular style of Mandarin that varies in pronunciation and grammar. Standard Mandarin, the official Chinese language, is based on the Beijing dialect.
Chinese Tonal System
All types of Chinese have a tonal system. Meaning, the tone in which a syllable is uttered determines its meaning. Tones are very important when it comes to differentiating between homonyms.
Mandarin Chinese has four tones, but other Chinese languages have more. Yue (Cantonese), for example, has nine tones. The difference in tonal systems is another reason why the different forms of Chinese are mutually unintelligible and are considered by many as separate languages.
Different Written Chinese Languages
Chinese characters have a history dating back more than two thousand years. The early forms of Chinese characters were pictographs (graphic representations of real objects), but characters became more and more stylized over time. Eventually, they came to represent ideas as well as objects.
Each Chinese character represents a syllable of the spoken language. Characters represent words and meanings, but not every character is used independently.
In an attempt to improve literacy, the Chinese government began simplifying characters in the 1950s. These simplified characters are used in Mainland China, Singapore, and Malaysia, while Taiwan and Hong Kong still use the traditional characters.