Type in what is css into google and the first result you get : CSS stands for Cascading Style Sheets. CSS describes how HTML elements are to be displayed on screen, paper, or in other media. CSS saves a lot of work. More over can control the layout of multiple web pages all at once.
Pretty cool hey, maybe you next question should be about seo and how not to spam a forum and what implications it has for your website and the forum as a whole