Even though they are combined in the master cylinder as a unit, you can think of the ABS (Antilock Brake System) and the power brakes as two separate systems.
1. Power Brakes
The red warning light staying on all the time tells you that there is a problem with the pressure in the power brakes system. This is a dangerous situation and the car isn't safe to drive. If you see the red warning light flash momentarily when you tap the brakes it is usually caused by a bad accumulator. The car can still be driven with caution but the problem should be fixed as soon as possible.
2. ABS
The yellow ABS light tells you there is a problem with the ABS. When anything in the ABS system goes wrong, even one wheel sensor, the ABS light will come on and the entire ABS will be disabled until the problem is fixed. The power brake part of the system will still function normally and you can still drive the care safely as long as the red warning light isn't on. You just won't have the anti-lock feature of the Teves brake system. The ABS makes the Reatta stop much quicker in a panic situation without sliding the wheels. It has saved my butt several times.
Something that should be noted is anytime the red warning light is on the yellow ABS warning light will come on within a few seconds. That doesn't always mean the ABS system has a problem. The problem that is causing the red warning light to be on should be corrected first before trying to get the yellow ABS light to go out.