Consistency is one of the principles that a designer must follow. Style Sheets help the designer so he doesn't have to apply manaully the same type of formatting over and over again thrughout the document.
Let me give you an example. You are designing a magazine promoting a certain item. You want to have all of your titles with a certain font, a certain size and a certain colour. You can record all of these information in a Style Sheet and then apply them to each title with a click. Now, let's say that you decide that the titles are too small and they all have to be made 4 points bigger. Well, you just go to your Style Sheet and modify the size of your font there and all of the parts of text with that Style Sheet will change in one go.
This tutorial is for users of QuarkXPress 5 and 6. If you want to know how to create Style Sheets in QuarkXPress 4 follow the instructions for QuarkXPress version 5. They should be the same mostly, there might be some differences, but if you get stuck you can contact me through my member profile in the forum and ask me any questions you have.
If you are an Adobe InDesign user, my article on how to make Character Style Sheets in InDesign outlines the procedure for that program.