HTML is Hypertext Markup Language and CSS is Cascading style sheets. Both are the major technology to build a website. We create structure of a website through HTML and CSS is for creating visual style, layout of a website. In simple word, HTML as the skeleton of the body and CSS as the skin and clothes on the skeleton. 
To build a website, we will need both technology together as CSS coding apply to HTML coding. CSS styling is applied on HTML elements. CSS enhances the colours, layout, and styling of the HTML webpage, making it look more beautiful and professional.

HTML is used to publish online documents with headings, text, tables, lists, photos, etc.
Retrieve online information via hypertext links, at the click of a button, Design forms for conducting transactions with remote services, for use in searching for information, making reservations, ordering products, etc. and Include spread-sheets, video clips, sound clips, and other applications directly in their documents.

Hyper Text: HyperText simply means "Text within Text." A text has a link within it, is a hypertext. Whenever you click on a link which brings you to a new webpage, you have clicked on a hypertext. HyperText is a way to link two or more web pages (HTML documents) with each other.

Markup language: A Markup language is a computer language that is used to apply layout and formatting conventions to a text document. Markup language makes text more interactive and dynamic. It can turn text into images, tables, links, etc.

Web Page: A web page is a document which is commonly written in HTML and translated by a web browser. A web page can be identified by entering an URL. A Web page can be of the static or dynamic type. With the help of HTML only, we can create static web pages.

CSS handles the look and feel part of a web page. Using CSS, you can control the color of the text, the style of fonts, the spacing between paragraphs, how columns are sized and laid out, what background images or colors are used, layout designs,variations in display for different devices and screen sizes as well as a variety of other effects.

There are few major benefits of using CSS:
•    CSS save times. Means, you can use CSS coding in other HTML pages/websites also.
•    If you are using CSS, page will load fast.
•    Easy to maintenance.
•    Provide superior design to HTML pages.