Cascading Style Sheets (CSS) is a declarative language that tells the browser how the webpage need to look like by declaring styles to selected elements to display them as you like. The browser then applies these CSS style declarations.
A style declaration contains the properties and their values.
/* The "font-size" property defines the font size, in this case 50px. */
/* The "color" property defines the color, in this case red. */
A CSS rule is a list of properties associated with the selector.
Is CSS only for HTML elements?
CSS is mostly used for styling HTML element but is also can be used with other markup languages such as SVG or XML.