CSS, Cascading Style Sheets, controls the visual presentation of HTML -- colors, fonts, spacing, and positioning -- using selectors that target specific elements, classes written as .classname, or IDs written as #idname. The box model describes how every HTML element is rendered as a rectangular box made of four layers, from inside out: content, padding (space inside the border), border, and margin (space outside the border) -- understanding this model is essential for controlling spacing correctly instead of guessing. 'Cascading' refers to how CSS rules can come from multiple sources, such as browser defaults, external stylesheets, and inline styles, with more specific or later-declared rules generally overriding earlier, less specific ones, which explains why a style sometimes 'doesn't apply' until you understand which rule is winning.