Kjonnnn,
Use layers where ever you want to place content on a web page. That's all a layer is, simply a placeholder for content. The advantage to using it is that you can specify down to the last pixel where you want your content to go. Once you have laid out your page in layers, the idea is that you convert it to tables for maximum browser compatibility.
Style sheets can be applied to layers, but layers can also be used without style sheets too. Just set the text font etc using the properties selector.
Hope this answers your questions.
Stuart