A style sheet is simply a text file that controls and manipulates the presentation of your content (which in many cases will be marked up in HTML format).
Style sheets can make your life much much easier because one style sheet can control multiple HTML files. Why is that so powerful?
Well, let's say that you wanted to change the background color and the text color of 500 separate webpages. Using HTML attributes, you would probably have to change 500 files; using one style sheet, you only need to change one file (i.e. the CSS file).
Using CSS to design your website will allow you much more flexibility, it will save time, make site maintenance easier, decrease download times, and best of all, you don't need an expensive WYSIWYG editor like Dreamweaver or FrontPage to make elaborate web pages.