Wikipedia defines a favicon (short for favorite icon), also known as a shortcut icon, website icon, tab icon, URL icon, or bookmark icon, as a file containing one or more small icons, associated with a particular website or web page. It’s a form of branding for a website.
Usually, favicon’s are in PNG format and sized at 16x16px or 32x32px.
Favicons are typically displayed in a browser’s address bar to the left of the name of the site. They can also be found to the left of the name in the bookmark bar of a browser.
Favicons also display on mobile devices. On Apple devices, custom favicon icons can display on users Home screens using the Add to Home Screen feature. These custom icons are usually 120x120px in size.