It's an interactive version of this. Cran is the repository that holds user-contributed software extensions for the R statistical language. Some of those packages were used to make this very graphic.
Instructions
You can zoom in and out with your mouse wheel, or move around the graph by clicking and dragging.
Hover over nodes to see packages' local networks---other packages whom they're directly connected to.
If you're lost, "Rescale Graph" will center you again.