Graphic layering like problem in game option

hi i'm new to using unity...So i thought of following a youtube tutorial...i used this tutorial.. http://www.youtube.com/watch?v=VV3sSM0gjQ8 it is a basic game with a car driving in the landscape... now when i press the play button... i get a layer like effect...on the different objects of the game.. for understanding this i've attached a screenshot of the problem i'm facing...! Thanks in Advance... :)