4X25MPH !
The 220 volt wiring going to your house will have a total of 4 wires in it:
1. A black wire that carries 110 volts,
2. A red wire that also carries 110 volts. The difference between the black and red wire is that they are 180 degrees out of phase, so that when the black wire is at +110 volts, the red wire will be at -110 volts and vice versa. The 220 volt appliances in your house, like your stove and electric dryer will have the heating elements connected between the black and red wires so that there would be a total of 220 VAC across those heating elements.
3. A white neutral wire. Think of the electricity coming into your house along the red and black wires and leaving along the white wire. If all the appliances in your house were pure resistors, like light bulbs, toasters, electric heaters and electric frying pans, then theoretically, both the voltage and current sine waves in the red and black wires would perfectly cancel each other out when the met at the white wire, so that there would be no voltage and no current in that white wire.
However, in practice many of the appliances in your house have "impedance" and that means that the inductors and capacitors inside the appliance cause the voltage and current in the black wire (and also in the red wire) to go out of synch so that the current isn't at a maximum when the voltage is at a maximum and vice versa. Your clothes dryer, for example, has an electric motor in it, which acts like an inductor or "coil" of wire, and the interference of the magnetic fields around each wire in the coil interfere with the current flow in all of the other wires in the coil, and that causes the current sine wave to lag behind the voltage sine wave in the wire coming out of the motor. Similarily, your TV set and computer monitor have huge capacitors in them, and in a capacitor the current is at a maximum when the voltage is CHANGING most rapidly, so that causes the current to be at a maximum when the voltage sine wave is at it's maximum slope at ZERO volts. Thus, that causes the current sine wave to be out of phase with the voltage sine wave in the wire coming out of your TV set or computer monitor.
Because of these "impedances", when the out of synch voltage and current sine waves from the black and red wire meet at the white wire, they most often won't cancel each other out perfectly, and you can actually get fairly high voltages in that white wire, like 40 volts or more.
4. The ground wire, which is always green or has no insulation at all.
Most of the electrical circuits in your house are 110 VAC circuits. Some of them are driven by the red wire, and some of them are driven by the black wire. The current from both kinds of circuits go back to the generating station along the white wire (or it's easiest to think of it that way). The ONLY circuits that will be connected between the black and red wires will be the 220 volt circuits to the 220 volt appliances, like stoves, electric dryers and electric water heaters. But, even the cables going to these appliances will need a neutral white wire inside that cable. That's because within stoves and electric dryers there are also 110 volt circuits, such as the one that powers the electric motor in the dryer and the ones that power the indicator lamps and convenience outlets on electric stoves.
The reason why the white wires are connected together in your electrical box is because the power is coming in through EITHER a red or black wire, going through the switch and then continuing through a red OR black wire to the light fixture and then back through the white wire to the generating station. That's why the white wires are connected together in the back of your electrical box.
If you have something like a light in the dimmer switch you're installing, then current has to flow through that light to the white wire in order to get back to the generating station. So, you would connect the white wire on the back of your dimmer switch to the white wires already connected together inside your electrical box. That is, connect all three white wires together.
I wrote up a blurb about basic house wiring for a lady who wanted to know how the new electric dryer she bought should be connected. If you want to know more about your house's electrical wiring, I can see if I still have it saved somewhere and I can post it in here for you.