in short, torque is the twisting force produce by the engine. A car engine can spin at different speed, thus RPM, but they might have different force at different speed.

Diesel engine has good torque at low RPM. Meaning they have good standstill acceleration, and they have the force to pull heavy loads. Petrol car usually have good torque at high RPM, thus can go higher max speed.

As for horsepower, it is just a number calculated from torque. it is basically calculated based on torque multiply by RPM multiply by 1/5252. So it means that if you car engine can give out high torque at high RPM, then your car engine has high horsepower.