Almost all the professionally developed websites have a Favicon. They are the web counterpart of desktop Icon.

A webmaster can install a favicon in a website by placing a favicon.ico file in its root directory. Eg. www.google.com/favicon.ico or www.kwelbytes.com/favicon.ico 

When we hit a URL the browser automatically try to fetch the favicon from url/favicon.ico. If it finds one it displays it on the address bar, else it displays the default image.

There are lots of websites which provide free favicons.  eg  freefavicon. Rather then downloading a prebuild favicon on can also build a custom one. You do that by providing a custom image which is converted to the ïco” format. Many websites provide this as free service. This service is also called Favicon generator.  eg favicon.co.uk,. There are some site which help create a favicon from scratch eg. favicon.cc