Wikipedia has a good article on quadtrees. The quadtree section in these slides is very good.
Here are some C implementations (found via Googling for quadtree c -"c++" -"c#"):
- http://hyantes.gforge.inria.fr/doc/quadtree_8c-source.html
- http://xw2k.nist.gov/dads/html/quadtree.html
- http://www.informatik.uni-ulm.de/acm/Locals/1999/src/quadtree.C
