On his homepage, Hanan Samet provides applets for visually learning how various spatial data structures work. They are great way to quickly get an intuition of how the various algorithms work. The "move" functionality is a particularly nice and interactive way of seeing how changes to the data affect the data structures.
(1) Go to http://www.cs.umd.edu/~hjs/
(2) Select "Online Demos"/"VASCO Spatial Index Demo"
(3) Scroll down; select "Lines"/"PM1 Quadtree demo"
(4) Draw some lines with the mouse
(5) Select "Operations"/"Move vertex"
(6) Move vertices around with the mouse and see how the quadtree changes.