Cascading Style Sheets were developed by Hakon Lie and Bert Bos working for the World Wide Web Consortium. The idea was to separate "presentation" -- the way the page looks -- from structure--the way the page is organized.
Using Cascading Style Sheets, you separate the style elements of the page (the font, size, color, spacing, etc.) and put them in a style section. Then if you decide later that you want to make a change, you only have to deal with one place, rather than hunting down all the places you used the old FONT and BASEFONT tags.
Later on in this book, we'll cover the various properties available with Cascading Style sheets, but this will do for an introduction.