This tutorial was first published in November, 2010. So you learned the base id, class, and descendant selectors - and then called it a day? If so, you're missing out on an enormous level of flexibility. While many of the selectors mentioned in this article are part of the CSS3 spec, and are, consequently, only available in modern browsers, you owe it to yourself to commit these to memory.
Let's knock the obvious ones out, for the beginners, before we move onto the more advanced selectors.