iOS, Android, and some other operating systems allow an app to volunteer to handle certain kinds of hyperlinks. Besides the usual http:// and mailto:// and ftp:// sorts of links, you can create your own links. I could, for example, create my own basil:// link. Indeed, this is the main avenue Apple provides for applications to talk to each other in iOS.
For more information…
Launching Your iPhone App via Custom URL Scheme by Rodney Aiglstorfer. He gives detailed steps with screen shots showing how to implement your own scheme.
Apple provides a brief intro to this topic, along with other Advanced App Tricks.
Apple documentation on the URL schemes built into iOS for calling the mail, phone, texting, Google Maps, Google YouTube, and iTunes apps.
