Mandarin and Cantonese are related languages that share common ancestry (generally believed to be Middle Chinese roughly 1000 years ago). The biggest perceived difference between them since they diverged is their phonology (i.e., how words are pronounced), but there are also differences in their vocabulary and grammar.
Your example of 苹果 is actually one where both the vocabulary choice and pronunciation actually didn't diverge much between Mandarin and Cantonese. Using IPA to more accurately compare the pronunciation, you can see that 苹果 is pronounced very similarly in both:
/pʰiŋ kuɔ/ (Pinyin: píngguǒ)
/pʰɪŋ kʷɔː/ (Jyutping: ping4 gwo2)
This is similar to how English and German (which are similarly related) have the same "word" for apple:
The Chinese writing system is somewhat unique in that characters generally represent words that are cognate among the different varieties of Chinese. Contrary to the notion that characters represent abstract ideas, instead Chinese characters generally represent the "words" that share a common origin (this isn't strictly true, but that's outside the scope of this discussion).
To give a basis of comparison, if English and German were also written with a morphemic writing system, the word for apple could conceivably be written using the same "character". Words that are cognate in both languages but where pronunciation has diverged further would also be written using the same character (e.g., English "tooth"
/tuːθ/ and German "Zahn"
/tsaːn/). Coming back to Chinese, 物 is an example where Mandarin and Cantonese pronunciations diverged more significantly:
/u/ (Pinyin: wù)
/mɐt/ (Jyutping: mat6)
Pronunciation isn't the only difference between them though. There's the common assertion that Mandarin and Cantonese are written identically but differ when pronounced. This isn't really true because the have also diverged in their vocabulary and grammar as well. Because modern written Chinese was standardized to be based on Mandarin, Cantonese speakers actually read and write in Mandarin (even though they may pronounce each character in the Cantonese reading). If they actually were to write the way they speak, the result would be different. This is a situation known as diglossia.
For instance, the third-person pronoun in Mandarin is 他
/tʰä/ (Pinyin: tā). This word is also pronounced very similarly in Cantonese:
/tʰaː/ (Jyutping: taa1); however, normally Cantonese speakers use a completely different word as their third-person pronoun, which is pronounced
/kʰɵy/ (Jyutping: keoi5). Because it's a different word altogether, it's represented using a different character, 佢.
This Wikipedia link describes more about the diglossic situation in Cantonese, along with a more extensive example: http://en.wikipedia.org/wiki/List_of_diglossic_regions#Chinese It also mentions the situation when the written standard was based on Classical Chinese rather than Mandarin.