TPU (Thermoplastic Polyurethane): The "soft" part. It is elastic and acts like a car's bumper, absorbing the initial vibration and slowing down the impact.
Polycarbonate (PC): The "hard" shell. It provides structural integrity and prevents the phone from twisting (torsion), which is a common cause of internal screen cracks.
Inner Linings: Some cases use advanced materials like D3O (a non-Newtonian fluid that hardens on impact) or honeycombed internal structures to create air pockets.