Cascading Style Sheets (CSS)
CSS is an acronym for Cascading Style Sheets. It was first invented in 1996, and is now a standard feature of all major web browsers.
CSS allows for developers to control how web pages look by “styling” the HTML structure of that page.
CSS specifications are maintained by the World Wide Web Consortium (W3C).
A good start is the freeCodeCamp curriculum Introduction to Basic CSS.
Another suggestion for beginners is W3C’s Starting with HTML + CSS teaches how to create a style sheet.
The site CSS Zen Garden is a great example how the same html can be styled to look totally unique.
For a demonstration of the power of CSS, check out Species In Pieces.