You should probably read up on it from a website dedicated to the topic. Volts and Amps are two different things. Combined, they produce power. They are also related through resistance and Ohm's law Voltage = Current * Resistance. The best analogy is the water analogy: voltage is like the potential energy the water has from one point to another (how high up it is relative to another point), and current is how much flow there is at a given point (how fat the pipe would be to transport that water between the two potentials). Resistance would be like turbines in the pipe that slow down the flow, producing power. Even a pipe without turbines (a wire) has resistance, though very small. The greater the voltage difference on either side of the resistance, the greater the current.
The 120V and 220V mains in your homes are alternating current (AC), not direct current (DC). DC is what most lasers run on. Unlike DC, in AC there's no "+" or "-" because the current is alternating back and forth. This is why you can plug most devices into the wall without needing to worry about plug "polarity." Some plugs do make a distinction between the two prongs. There is a distinction, which you can read elsewhere; however, it is not like "+" and "-" on a battery.
How do you use AC? Well, you can convert it to DC and use it. Then you'll have current flowing in one direction. You can also take advantage of the alternating current to do things like spinning a motor, since the motor alternates as well. Or maybe you could power a light bulb, since the tungsten filament acts as a resistance and will produce heat and light so long as there is current.
