A stylesheet is a set of instructions that tells a web browser how to display the design elements on a web page. Normally, you'd have to create a stylesheet by hand when you build a website, but Office Live Small Business creates one for you automatically. As a matter of fact, almost every setting that you set in the Site Designer gets translated into the instructions that go in your site's stylesheet. When you set your site's default font to be Georgia, for example, Office Live Small Business created an instruction in your stylesheet.
Each such instruction is called a style. The good news is that you can tweak, or even over-ride, the styles that Office Live Small Business generates automatically for a more fine-grained control over your site's look and feel.
A complete course on stylesheets is way beyond the scope of this book, but I'll give you a taste of what you can do with custom styles.