CSS (Cascading Style Sheets) is a styling language that controls the visual presentation of web pages. While HTML provides the structure and content, CSS determines how that content looks - from colors and fonts to layouts and spacing.

Key things CSS allows you to control:

• Colors and backgrounds - Set text colors, background images, and gradients
• Typography - Choose fonts, sizes, spacing, and text effects
• Layout - Position elements, create columns, and control spacing
• Responsive design - Make websites adapt to different screen sizes

CSS works by selecting HTML elements and applying style rules to them. These rules can be placed directly in HTML files, in separate .css files, or even inline with specific elements. The "cascading" part means that multiple style rules can apply to the same element, with specific rules overriding more general ones.

For shared hosting users, CSS files are typically uploaded to your web server just like HTML files. Most hosting control panels include file managers where you can easily edit and upload CSS files to customize your website's appearance without needing advanced technical skills.