The Hall effect allows us to find out whether charge carriers in a coductor are positively or negatively charged. A circuit component that makes use of this is the Hall sensor.
Hall sensors are fabricated with a p-doped semiconductor (so that holes flow from positive to negative). If a magnetic field is applied, the holes will experience a force to move to the top and electrons to the bottom. This effectively produces an internal electric field, such that the electric and magnetic force cancel out. So the current goes straight through.
We can measure the voltage between the holes at the top and electrons at the bottom, the Hall voltage.