Os is in short for operating systems - A software to interface between the end user and the hardware. A software is normally divided into layered components to undertake various tasks at their levels.Its job is to hide all complexity involved from the end user.

GUI stands for Graphical user interface - Earlier OS didnt have a GUI - Fox example the legacy OS MS DOs - While later OS like Windows ar famous for their GUI. The most recent OS in development is Windows Vista after older Windows,Solaris,Linux and many other OS.