These three types of software work together to structure your data, define how it looks, and how it should behave.
Hypertext Markup Language
HyperText Markup Language is the coding language used to structure website content. HTML defines a thing as, for example, a header, navigation menu, or an image.
HTML is used by page readers to help the visually impaired access websites.
Cascading Style Sheets
Cascading Style Sheets are a coding tool used to define the way things look. Color, font, spacing, etc. are all specified using CSS. Once HTML tells the web what a thing is (i.e., a heade), CSS tells it how it should look.
Do your images have borders? What color are they? Do they have rounded corners? Drop shadows? What fonts are your headers and body copy? What point size are they? Are they left-justified, centered, or right-justified? These things are all controlled by Cascading Style Sheets.
The same image with three different border styles. In this case, the HTML (the image, or data) is the same but the CSS (styles of presentation) are different.