Basically very fast computer processors and such monitoring forces, slipping, etc. occurring while driving and apply single or multiple wheel braking, regulate power application, etc. to keep the car in line and steady. Different from the also included traction control which the main purpose of is to control power delivery at the drive wheels alone via braking or power reduction when slipping is noticed, versus controlling the stability of the entire car.
My ESC actually activated today due to the snow... I have 14" of snow so far (powdery) and just the standard all-seasons on.. I swear you'd thing this car had AWD. Only sliding I encountered was braking (But that's common in snow, especially roads that haven't been sanded yet).
I encountered this last night as well while driving our Honda Pilot in the snow.
Turning a corner in town at about 20mph, I intentionally stayed on the gas to see how well the all wheel drive would react. It was tracking well up to the midpoint of the corner at which time the system sensed the back end coming around too quickly.
Even though I never hit the brake and didnt steer into the slide, the system utilized the abs system and slowed the vehicle until the angle straightened out. The system feels and sounds just like the vibrating you get when abs comes on.