Thermal imagers can "see" through walls, although they do not give a perfect or even very accurate picture of what is going on inside.
Law enforcement can not use these devices, and by the ruling, any new technology that acts in a similar way, on house without a search warrant as per a recent Supreme Court decision. This would not prevent them from using it if a warrant had already been obtained or if they were performing a hostage rescue.
The thermal imager shows differences in heat. A person inside of a building can usually be located using one of these. The are also useful for finding hidden compartments on vehicles and in cargo, but usually because large quantities of dope have a different heat signature than their surroundings.
They do not look like the stuff Hollywood shows, and require a trained, skilled operator to be used effectively. They run about ten grand.