M12 runs on 12 volts, M18 runs on 18 volts. Some tools offer both Fuel and non-Fuel versions for M12 & M18, they are all brushless. But take note of non-Fuel brushless tools that are exclusive for 18v tools.
This brushless technology is more advanced, offering more power, longer lifespan, and reduced battery drain technology. But for me, fuel models are way better, providing better quality and performance than any standard brushless tool. Also, non-fuel seems to be getting phased out by the company.
There are certain tools that is 12v and 18v versions. Be wary that they are different and they use different batteries.