CSS, or Cascading Style Sheets, is a type of programming language and style sheet language. Essentially, where the purpose of HTML is to mark up the text being put on a website (such as through font and color changes), the purpose of CSS is to give that text structure, whether it be via a website on your desktop computer, your mobile phone, or elsewhere. This entails providing not only the structure for the HTML to be placed, but also to design your website however you deem fit (though we do strongly recommend that how you see fit to design it is actually a good idea).
Much like HTML, CSS is a necessary component of any functioning website. If the website did not have a CSS, at that point it simply does not exist. Even if you only stick with the default CSS for a website, that is still a CSS. Without HTML, you won't have the text itself. Without CSS, you won't have the structure to display HTML's accomplishments.
Does every website use CSS?
CSS is required for any and all websites to function normally, as it is a basic building block of any website (and indeed, every website). If you are designing your own website, it would be vitally important that you come to better understand how CSS works, and how it can be made to work for you.