In CSS, margins are the transparent space around an element’s content, pushing other elements away. They are specified using the margin property (for all sides) or margin-top, margin-right, margin-bottom, and margin-left (for individual sides). Values can be set using lengths (e.g., pixels, ems), percentages (relative to the parent’s width), or the keyword auto (for automatic calculations, often used in centering). Unlike padding, which adds space within an element’s border, margins affect the element’s position relative to other elements on the page.