All bicycle helmets sold in the US must meet Consumer Product Safety Commission standards to be legally sold in the US. This ensures a basic level of protection. More expensive helmets are generally lighter, better ventilated, and perhaps more aerodynamic.
The latest new bicycle helmet technology which is now widely available is called MIPS which stands for Multi-directional Impact Protection System. This allows the helmet to rotate slightly on impact, absorbing some of the rotational forces that might otherwise increase the risk of traumatic brain injury.