What is Html?

Html is the language of the web. Every web page in the Internet is writen in HTML, because it is the only language that browser understands to show the content in the web.
But wait, Isn't HTML written in English language? So, how can it be a different language?

When you upload a page to the web server, and it is downloaded (viewed) by client on the other part of the world.
Imagine how does his browser knows exactly a page is structured in a certain way and show it in a way that you have declared it so.
That's exactly what HTML do. It is a way of communication for the browsers to structure (as DOM) and show it to the users so that they browse the web conveniently.

Browsers like Google chrome, safari etc. do not understand English Language , nor follow their syntax. However they do understand commands  provided to them (as HTML tags).
  

What is CSS?

You might be wondering - "If HTML is all browser needs to display a web page, then why do people talk so much about CSS?
Well, HTML alone can display a web page. ( It's true ).
But, if you need a web page well designed and attractive, Html is not enough at this time.
Css plays an important role in styling a page in order to make a site look like however a webmaster wants it to be.

Html vs Html5
CSS vs CSS3