AbhijeetMajumdar

A quadtree is a tree data structure in which each internal node has exactly four children. Quadtrees are most often used to
partition a two-dimensional space by recursively subdividing it into four quadrants or regions. The regions may be square or 
rectangular, or may have arbitrary shapes. 

More on 
https://en.wikipedia.org/wiki/Quadtree

Thanks to Jim for such an excellent visual representation on http://jimkang.com/quadtreevis/