Written Mandarin vs Cantonese

[Sun Wukung]

I really don't get the difference between the 國語 (guoyu or national language of China) and the Cantonese spoken in mostly Guangdong. When you see it written there are only some noticeable differences like the use of traditional characters for the written aspect as well as special use characters. That leaves the inevitable question of what is up with the two written languages and what is not the same. If someone could clear this up, it would be 很好(hen hao=very good）.
Xiexie.