Collision detection

http://en.wikipedia.org/wiki/Collision_detection

http://stackoverflow.com/questions/4981866/quadtree-for-2d-collision-detection

http://stackoverflow.com/questions/4434335/quadtree-for-2d-collision-detection

http://gamedev.tutsplus.com/tutorials/implementation/quick-tip-use-quadtrees-to-detect-likely-collisions-in-2d-space/

http://gamedev.tutsplus.com/tutorials/implementation/make-your-game-pop-with-particle-effects-and-quadtrees/