In addition to registered users being able to modify the way that various parts of the site are displayed from within their accounts, note that some features can be turned on and off or collapsed. For example, if there are forum categories that you're not interested in, you can ignore them through your settings, or collapse the display of the categories. Look for a square button with an up or down arrow to collapse (down) or expand (up) like this. Some features are normally collapsed, like the 'Other options' available below this post as I write it, and they work the same way.