CSS borders are styling elements used to create a visible boundary around an HTML element. You can control the border's width, color, and style using properties like `border-width`, `border-color`, and `border-style`. For example:
```css
.element {
border-width: 2px;
border-color: #000;
border-style: solid;
}
```
This creates a solid black border with a width of 2 pixels around the element. There are other border styles like `dotted`, `dashed`, `double`, and more. You can also use the `border` shorthand property to set all border properties in one line.
Remember to use these properties within the context of a CSS selector to target specific HTML elements for styling.
