What's the difference between Autocad and Archicad ? and what is the relationship between them ?

ARCHICAD IS USED FOR 3D MODEL PRESENTATION, LIBRARY PARTS ARE VERY HELPFUL TO PRESENT THE 3D MODEL IN RENDER VIEW, AUTOCAD IS A SOFTWARE USED FOR DRAFTING KNOWLEDGE IN CAD. WE CAN INSERT THE CAD FILE IN ARCHICAD AND DRAFT THE PLANS AND PREPARE 3D MODEL.

in archicade we can use the 3D in our conception and in autocade we use just the 2Delements the relatioship between them is that the archicade can complet what we do in autocad

Most AutoCAD users, even architects, mostly use the software for 2D drafting. ArchiCAD (or now ARCHICAD) on the other hand is built for BIM and Virtual Building Modelling.

No, it is a competely separate package. It can be purchased in a suite with AutoCAD, and now with ACA in the new release if you need to keep AutoCAD around but it is not needed to run the software. The reasons you may want the suite with AutoCAD or ACA is to work with legacy files, or some detail in AutoCAD because it is better/easier (in my opinion) that in Revit. Either way you don't need it In a simplified way think of AutoCAD as lines, ACA as objects (walls, doors, windows, etc.) and Revit as a model. Revit is AutoDesk's BIM solution for architecture