The dialects from the seven groups are quite different. For example, a Mandarin speaker in northern China usually understands little Cantonese, but a non-Mandarin speaker usually can speak some Mandarin with a strong accent. This is largely because Mandarin has been the official national language since 1913. Mandarin or Putonghua is mainly based on the Beijing dialect. Despite the large differences among Chinese dialects, there is one thing in common for them -- they all share the same writing system based on Chinese characters.
A distinguishing feature of the Chinese languages is tonal. Mandarin has four tones and Cantonese has more than four tones.