mandarin is not equal to standard Chinese.
the dialect of mandarin around North of China,especially in Beijing, is almost equal to standard Chinese,which we use to communicate with each other all over China when we can't understand their own language.
mandarin has its own dialect such as the one spoken in Southwest.
Cantonese is a language,or a dialect as we often say in China, used in some region in South of China.
The spokers of the two languages,mandarin and cantonese, can't understand each other if they can only speak their own language.
But they use the same writting system,so that they can communicate by writting if the spoker of Cantonese want.
Cantonese is different from mandarin in pronounciation,tone and expression.But the vowels and consonant may used in Cantonese can be implied according to mandarin sometimes.