There are hundreds of dialects in China, if not thousands. But all mainland Chinese, as well as Taiwanese, hongkongese and overseas Chinese speak Mandarin to each other, in addition to their own dialect.

Mandarin is the official Chinese. It is called "standard language" in Chinese ( Pu Tong Hua).

When people talk about Chinese, they mean Mandarin by default. It is a bit like when you say German, you mean high german, not swiss german by default.