Member Rara Avis
Nicole has you covered, Becky.
An example of hierarchical is the main Passions site. You start at the top (main page), then go to a category (Poetry Buffet), then go to a subcategory (Christian Poetry). To go from Christian Poetry to Lost Romance, you have to go back up the tree (to Sad Poems) and then down into a new subcategory.
Once you get into a category or subcategory at the main site, it becomes an example of linear - because all the poetry is laid out in a straight line and, indeed, every poem has a link going to the "next" poem.
The main site, obviously, is a combination. So are the forums, now that we have MetaCategories, and that similarity is not a coincidence. IMO, it's the only effective way to build a site of any nontrivial size, and was our biggest lack in the old software.
Unfortunately, navigating a "tree" is only intuitive if you can see the tree structure and realize other nodes exist. That's why I added a visual at the main site a few months ago ( http://netpoets.com/poems/ ) and why you'll likely see one here before we grow much larger. Intuitive, however, doesn't necessarily mean easy. Both the main site and the forums currently have only two levels before they go linear, and that makes constantly going up and down the tree tolerable. Add a third level and you enter a whole new realm of complexity - because "easy" is only going to be accomplished by cross-linking the lower nodes. But we'll save that one for another discussion.