If you have a cable feeding red, black, and white to every outlet, this is called "shared neutral", "split phase", or "multiwire branch" wiring.
That system allows two 20 amp 120 volt circuits to be fed using just three wires from the breaker box instead of the four that two independent circuits would require. The neutral wire is shared between the two circuits.
It is common to have the top and the bottom portion of an outlet on the separate hot wires (this requires breaking the common tab off of the hot side of the outlet, and is not possible with a GFI outlet), or some outlets can have both sockets fed from one hot, and other outlets on the string use the other hot.
In all cases the neutral side of the outlet is the white wire. You cannot daisy chain GFI outlets on this system, because the shared neutral wire carries an unbalanced current versus one of the hot wires, and a GFI outlet is designed to detect unbalanced current and trip.
The red and black wires are both hot, and they must be on opposite "legs" of the feed from the utility. Two-pole breakers should be used in the breaker box to enforce this. Two single pole breakers could be used, but then they must be inserted into the box in slot positions so that they are fed from opposite bus bars in the back of the box.
It is very important that the two hot wires be out of phase. When two 120 volt loads with the same amperage are connected to opposite hot wires, the current flows between them and no current need flow in the white wire. The worst case has 20 amps of loads on one hot wire but none on the other, in that case the 20A returns through the white wire.
If the two hot wires are improperly connected to the same phase in the breaker box, the two loads will need to return their combined current (up to 40A) through the white wire. That will overheat the wire and is a fire hazard.
