Confused about Categories and Subcategories? Don't go nuts about it. It's just a new method of organizing.
Put photos in galleries. Those galleries can then be organized with categories and subcategories.
Category > subcategory > gallery > photos
You can display either galleries or categories on your SmugMug homepage.
Say you are a wedding shooter. Simply create a new subcategory for each wedding. So, Smith-Jones gets a subcategory. In that Subcategory, you can now add galleries, for each part of the wedding: Before, The Ceremony, Formals, Reception, etc. The way it looks is just like this:
Here is the subcategories page, all your weddings would be here. Click on one of the wedding couples, and you can see how the galleries are underneath. Clear as mud?