CSS: Creating a Drop Down Navigation Menu

Every tutorial I've done on navigation bars or menu's has been the normal one level navigation bar, but when you build a website you may feel the need to create sub categories, for example my blog houses various tutorials which are stored in categories, I have a drop down navigation bar on the main link, which is tutorials which then displays sub catergories and then the specific catergory e.g. Tutorials > XHTML > Basics in that example there are three levels. You can create navigation bar level