The Contacts API provides URIs to access entries in a person's Address Book. Entries typically consist of email addresses for contacts that the address book owner has chosen. Using the Contacts API, you can allow users to quickly access their contacts from a website or a handheld device.
For example, suppose you want to send eCards to friends for Christmas. If the eCard company uses the Contacts API, you can access your Address Book to get your friends' email addresses directly from the company's website or from an iPhone app. You can even update the entries for your friends or add a new contact.