apple-touch-icon
(apple-touch-icon.png) Apple's default icon (image) used to represent a website, e.g. when saved as a bookmark or on the home screen of an iOS device such as an iPhone or iPad.
Apple's scheme allows a site to offer images of different sizes so the client can choose the most appropriate one according to its screen size and resolution.
Apple devices and applications completely ignore the favicon.ico de facto standard which, while somewhat quirky in its use of the ico format, has been pretty much universally adopted elsewhere. Conversely, apple-touch-icon.png will be ignored by non-Apple devices, possibly because its 16x16 resolution would look pretty shabby on most smart phones.
(https://developer.apple.com/library/ios/documentation/AppleApplications/Reference/SafariWebContent/ConfiguringWebApplications/ConfiguringWebApplications.html) Apple documentation.