Cascading Style Sheets is a style sheet language used for the presentation and styling of a document that is written in a markup language such as HTML or XML. CSS is where you add color, fonts, and animation, etc. Basically CSS makes websites more appealing to people.

1. CSS Cascade:
   The cascade is the system for solving problems where multiple CSS rules apply to an HTML element. CSS Cascade takes care of the position and order of appearance, the specificity, the origin and the importance.

2. CSS Selectors:
   It is a decoration of elements and other terms that tell the browser which HTML elements should be selected to have the CSS property values inside the rule applied to them.

3. CSS Specificity:
   It is the system used by browsers to determine the CSS declaration that is the most relevant to an element, which determines the property value to apply to the element.