If you ever did some job linked with google map (f.x. show markers with popup) you probably noticed problem with huge amount of markers (f.x.1000 markers). It does slow performance of webpage + it looks not very nice on UI (just mass of markers on same place). There is solution for such situation: MurkerClusterer, it allows to join markers into cluster with possibility to have different cluster on different zoom.
Here is an article about it - markerClusterer solution to the Too Many Markers problem
For those who works with Google Maps API this article is mandatory to read.