First major difference is pronunciation between Cantonese and Mandarin
Cantonese has at least 6 tones (some said 9 tones) while Mandarin has only 4 simple tones plus a neutral tone, which does not exit in Cantonese

Here’s the tone comparison between Cantonese and Mandarin:
Cantonese:<a target="_new" href="http://www.mdbg.net/chindict/chindict.php?page=chardict&cdcanoce=1&cdqchi=fu1%E5%A4%AB++fu2%E8%99%8E+fu3%E5%AF%8C++fu4%E6%89%B6+fu5%E5%A6%87+fu6%E8%B4%9F%0D%0Afrom+ChineseBay.com+CantonesePod.com&cddmtm=0&cddytm=0">fu1夫 fu2虎 fu3富 fu4扶 fu5妇 fu6负幸福=happy/happiness
Note that Mandarin xing4fu幸福 is pronounced as hang6fuk1幸福 in Cantonese</a> (click for details)
Mandarin: <a target="_new" href="http://www.mdbg.net/chindict/chindict.php?page=chardict&cdcanoce=1&cdqchi=fu1%E5%A4%AB+fu2%E7%A6%8F+fu3%E6%96%A7+fu4%E5%AF%8C%2F%E8%B4%9F%0D%0Afrom+ChineseBay.com+CantonesePod.com&cddmtm=0&cddytm=0">fu1夫 fu2福 fu3斧 fu4富/负</a> xing4fu

Mandarin (or <em>putonghua</em>) is the standard and official language in mainland China.