SiteBrowser's purpose is to displays items that are organized in a tree like structure, allowing user to browse through them and click to get a document, associated with an item. So, each item have visual components, optional URL of associated document and optional "target" - window in which associated document will be displayed when user click on the item. If you don't specify target, it's taken from TARGET parameter. All this components of an item and tree structure itself are defined in a separate text file (see tree file structure for more detail), while applet's appearance and behavior are defined through parameters within your html file. This concept both simplifies defining tree structure and html writing. You may try out different feels and looks without changing the tree structure. You may generate tree structure online by some other program which won't interfere with the feel and look you defined.
SiteBrowser is very customizable, there are plenty of parameters which affect it's behavior and appearance. Look at applet parameters for more detail. Some parameters are of "global" nature while others are on a feature-per-level base. Level of an item is it's depth in the tree structure. Top of your tree structure have level #1, subitems of a top-level item have level#2, etc. For example, you may specify different fonts for different levels, different vertical spacing and different icons and shapes.
Now more detail about item's visual components. There are two of them, icon(shape) and content. Components are displayed next to each other. Example of a shape and content is a dot and a text right to it. Usually you use just text as content, it's defined in the tree file structure. It's font is taken from applet's FONTn parameter (level-based feature). But you may also specify your own images for the custom content. Same applies to icons and shapes. You may use predefined shapes which you specify in applet parameters(level-based feature). This shapes are smoothly rendered internally by the applet with size accommodated to item's font height. SHAPEn parameter is taken for items which have subitems and thus can be expanded, ICONn parameter is taken for leaf items which can't be expanded anymore. You may use custom icons and shapes which you provide your own images for. Specify custom icon/shape for an item in the tree file structure, this overrides ICONn and SHAPEn parameters. For more detail see custom items.