Quartz é a camada de gráficos de baixo nível, e o processo WindowServer gerencia as janelas na parte superior isto. Ambos são softwares proprietários da Apple.
De aqui :
Two interacting systems create and manage Cocoa windows. A window is created by the window server. The window server is a process that uses the internal window management portion of Quartz (the low-level drawing system) to draw, resize, hide, and move windows using Quartz graphics routines. The window server also detects user events (such as mouse clicks) and forwards them to applications.