The word "dialect" means something different when applied to Chinese than it does for other languages. Chinese "dialects" are mutually unintelligible, as different as, say, Spanish and French and even English, which we would call "related languages" rather than "dialects".
However, while there are different spoken dialects of Chinese, there is only one form of written Chinese, with one common set of characters -- mostly. An exception arises where in some spoken dialects, for example Cantonese as used in Hong Kong, more informal phrasings are used in everyday speech than what would be written. Thus, there are some extra characters that are sometimes used in addition to the common characters to represent the spoken dialect and other colloquial words. One additional complication is that mainland China and Singapore use simplified characters, a long-debated change completed by the mainland Chinese government in 1956 to facilitate the standardization of language across China's broad minority groups and sub-dialects. Hong Kong, Taiwan, and many overseas Chinese still use the traditional characters.
About one fifth of the people in the world speak some form of Chinese as their native language. It is a tonal language that is related to Burmese and Tibetan. Although Japanese and Korean use Chinese written characters, the spoken languages are only very distantly related to Chinese. Also, the unrelated Vietnamese language has borrowed many words from Chinese.
Yet other languages may've borrowed so much from Chinese that they've come to be considered Chinese languages. Hokkien/Taiwanese/Ban-lam-gu and Teochew could have "become Chinese" this way.
Note that travellers headed for Guangdong, Hong Kong or Macau will almost certainly find Cantonese more useful than Mandarin.