What is Twig, and PHP Templating

I am completely confused what Twig is. Is it PHP or HTML? Also, what exactly is "PHP templating"? And would I need it (or PHP at all for that matter) to create a website which simply displays information to the user? Couldn't I just use HTML, CSS and JS for that?

Also, I have seen website source code with no .html files at all only .twig files, how does this work?