The app is called Yarn U and it is available through the Apple app store . This app is quite a reference for popular yarns available today. (am sure more information is added and will keep getting added over time about more yarns.
The app is developed by Mary Beth Klatt who is a yarn collector and has written another app called Yarn U for sewing enthusiasts.
There are a number of ways to navigate the app. Simply through the table of contents, alphabetical index of yarns by name or yarn company, or filter by yarn weight. To get started there is a small manual for the app users built into the app.
Another great feature I found was that if you allow the app to use your location, it also gives you the address of the nearest store where you can buy it. However this feature is currently available only for USA based location. There are also pointers on an area map for nearest locations to buy the yarn.
All in all it is an app with a great potential and already very useful. Head over to the apple store and download it today.