Nearly the same as a Octree but instead of dividing the space in cuboids in 3D it’s rectangles in 2D space. This is useful for rendering terrain and you want to cull a lot of the terrain fast.

A visual demonstration of a quadtree used for fast collision detection:
http://lab.polygonal.de/2007/09/09/quadtree-demonstration/

Introduction to quadtrees and sample code:
http://www.gamedev.net/reference/programming/features/quadtrees/

More info about quadtrees, including source:
http://www.kyleschouviller.com/wsuxna/quadtree-source-included/