iCloud includes push email, calendaring, and contact services. That means whenever you get an email, it's immediately made available on all your devices. Whenever you make or change an appointment on your iPhone, it instantly shows up in your Calendar app on your iPad and Mac. Whenever you add or change contact information, its reflected in the contacts on all of your devices. iCloud will also sync Bookmarks, Notes, and Reminders. You're prompted to set all this up when you first set up your iPhone or iPad, but if you chose not to, it's never too late. You can also set it up at any time!