What are Favicons?

Favicons are small 16x16 icon files that are displayed next to the URL of your site in a browser's address bar. Additionally they're often displayed next to the name of your site in a user's list of open tabs and bookmark listings making it easier for the user to quickly identify amongst other sites.

What are App Icons?

App Icons are the images you press on your smartphone to launch an application. As newer phones are released with higher resolution screens, higher resolution app icons are needed. Developers still want to maintain support for the older phones with lower resoltion so when you create an app icon you need to create several size variations of the same image. This is true for all smartphones like the iPhone and Android, and even tablets like iPad.

more: Dan's Tools