Glossary:
CSS
CSS is an abbreviation of Cascading Style Sheet. CSS are declarations that specify how web page elements are displayed by a web browser.
CSS follow standard notation and can be included in the head of a web page or can be written in an external style sheet file (usually named style.css in the root web folder).
Multiple pages can reference the same style sheet, reducing the amount of code and allowing fast editing of all style elements in a web site (by editing the one style sheet).
A web site may have multiple style sheets to describe how it should be displayed on screen, for printing and for accessibility.