Injector driver

Hey ben , i had a few questions listed below

1. what is low impedance and high impedance injector ?

2. What type of injector are used in OEM'S (eg: high impedance or low impedance) and what type of injectors are used for motorsports application

3. Does low impedance injector require an external driver to run or high impedance injector require an external driver on a basic stand alone ECU such as megasquirt