Torque steer, what causes it?

My Dad recently changed cars from an old Volvo 940 (rear wheel drive and a wheezing 2 litre petrol engine) to a Saab 93 that has front wheel drive and a 2.2 litre turbo diesel engine. “Testing” his new car i.e. showing me how fast he could drive, Dad said he could notice the difference in fwd and rwd when he accelerated hard. Even on straights the Saab did pull slightly on the steering. I think this is torque steer as its mentioned disapprovingly in car mags I read. So what is it and why does it happen?

This recent Slate article mentions it.