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.
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.
Although many modern web browsers support favicons saved as GIFs, PNGs or other popular file formats all versions of Internet Explorer still require favicons to be saved as ICO files (a Microsoft icon format). This tool provides an easy way to convert any GIF, PNG or JPEG to ICO which is supported by all modern web browsers. It also enables you to create favicons from scratch via a handy online editor. Additionally the editor lets you manually tweak generated favicons to ensure the best possible result.
After generating a favicon with this tool download and save to the root directory of your site. Then include the following code in the head of your HTML document.
<link rel="shortcut icon" href="/favicon.ico" type="image/x-icon">
<link rel="icon" href="/favicon.ico" type="image/x-icon">
