Intake air temperature sensors are typically located in the intake manifold. The sensor is positioned so that the tip is exposed to air that is entering the engine. Some engines use mass airflow sensors to measure the volume of air flowing into the engine and have a temperature sensor too.
Intake air temperature sensors are essential for a vehicle's engine to estimate the correct mixture of fuel and air. Cold air is more dense than hot air, therefore, the temperature of the air is vital for the car's computer to properly gauge and distribute fuel, and allow combustion.
V-6 and V-8 engines with split or separate intake manifolds will have two intake air temperature sensors as well.