What six said. iCloud syncs your mail with all of your devices. If you send/receive an email on your Mac at home, it will also appear on your iPhone, iPod Touch, iPad and vice versa. You do not have to use the web interface at all if you don't need to (like if I were away from any of my devices and needed to access it from a family member's/friend's computer — Note: I still don't have any iDevices, just my home Mac.).