Favicon refers to the small icon we see on the top right corner of the browser. It is used to uniquely identify a particular website. It is a minor but rather important feature for a website to have. It is also quite easy and quite complex to implement. Considering the way web browsers treat favicon differently.
For example, Jonathan T Neal in his excellent post “Understand the Favicon” writes
This really depresses me, because Chrome, Firefox, Opera 7+, and Safari 4+ all accept the PNG favicon, but Chrome and Safari will opt to use the ICO favicon when both are presented, regardless of the order in which they are declared. On the other hand, Internet Explorer does not support PNG favicons, but it will ignore the PNG favicon and use the ICO favicon, regardless of the order in which they are declared.
Even in simple implementation like favicon, browsers behave differently. In his article, Jonathan further explain the subject of favicon and its implementation. I strongly recommend reading his article to gain an understanding of favicon.
So, if you are interested to learn more about favicon, do visit Jonathan T Neal article “Understand the Favicon“.