Tag shadow is a an intersection of art and geekiness. The goal is to visualize a group of books into beautiful and unusual groupings. The following description is a work in progress and I'd very much like input on how clearly this explains the concepts.
Two Dimensions
To understand what's going on, think about 2 of your favorite things. I'm going to start with science fiction and vampires.
One way to organize all your books about Science Fiction and vampires would be to place the science fiction on one bookshelf and the vampires on another bookshelf.
Obviously the problem is that some books are about both science fiction and vampires to one degree or another. Lets put all of those books on one book shelf together.
Place the books that are about MOST about vampires on the top shelf and as you move down the shelves add the books that are less and less about vampires.
Now look at each of these shelves individually. From left to right, order the books by how much they feel like science fiction.
Once you're done you have a rather odd book shelf, but it serves one purpose perfectly. It helps you find exactly the mix of science fiction and vampires that you're in the mood for at a given point. And if a friend comes over, they can browse around until they find a book they've read and liked and it's likely they'll like the books shelved near by.
Three Dimensions
Now lets assume you're organizing a convention for aficionados of science fiction, monsters, and magic. In the dealer room you decide to arrange the tables such that the science fiction is more concentrated at the front and monsters are more concentrated to the left. So, if you're wanting to find a table with the a lot of science fiction but no monsters, you'd make your way to the front of the room and to the right.
At every table we're going to organize stacks of books such that the more magic they have in them the higher they are in a stack. Some vendors might use a shelf for the same purpose, but the idea is the same.
What we now have is a way to organize 3 ways of describing books using the 3 dimensions we're used to interacting with.
Computer Screens Are Flat
We use many different ways to visualize 3 dimensions on a flat display. Think about the previous example, but imaging it in a weightless environment where books could use the entire height of the room to describe how much magic they had in them. One way to understand in 2 dimensions what's happening in 3 dimensions would be to shine a bright light through the room of books and see what the SHADOW looked like on the wall.
You'd have to move the light around a while until you found a direction which best captured the 3 dimensional groupings in the shadow. I'm not going to go into details here, but that's a problem that's got a fairly good mathematical solution. That solution isn't limited to 3 dimensions. In fact, we can figure out a 2 dimensional plot derived from hundreds (or even thousands) of descriptive tags.
Tags + Shadow
I decided to call the groupings of books that I derive from all the tag information a TagShadow. Hopefully you have a general idea what I'm trying to accomplish at this point. Click on tags, hover over points and just explore.