The circle of Willis is an anastamosis where the posterior circulation (those arteries derived from the vertebral arteries) and the anterior circulation (those arteries which arise from the carotid arteries) come together.

 The carotid arteries enter the skull along the anterior aspect of the neck.  The internal carotid artery (ICA) typically terminates in a bifurcation into the anterior cerebral artery (ACA) and the middle cerebral artery (MCA).

The vertebral arteries fuse to form the basilar artery .The basilar terminates when it bifurcates into the posterior communicating arteries.