In USA 240V comes into your home even though most of your equipment is 120V. I say most because your clothes dryer for one, if you have an electrically-heated one, most likely runs on 240V. The answer to your question is that 2 legs (power lines) come into your house. One leg is colored Black, the other is Red.
Each leg is 120V to common (ground). The voltage is AC (Alternating Current) so it is always going from +120V to -120V on each leg. But at opposite times, so that at some point in the cycle one leg is +120V while the other leg is -120V. And if you measure the voltage difference between +120V and -120V you get 240V.
The US wiring system works like this: to turn on your lights you switch into one leg, the current goes through your wiring to your lights and back to common. But your dryer switches into one leg and returns the current to the other leg. If you look in your breaker box you see breakers on both sides. Looking straight across, these breakers are on the same leg (L1). The next set down the panel are on the opposite leg (L2). Adjacent breakers all the way down the panel are on opposite legs. A two pole breaker spans these adjacent legs and that is how you arrive at 240 volts. Your dryer is connected to a two pole breaker that taps into both Black and Red legs.
To accurately specify a voltage, you have to specify two points to measure, and the voltage is the difference between the two. If only one point is given it is usually implied that the second point is a grounded or 0 volt point. In the U.S., the three wires that enter most homes can be thought of as at +120V, -120V and 0V relative to ground. The 0V line is the "grounded" conductor, sometimes also called "neutral" and must be well connected to the earth where the wires enter the building (and no where else!) The +120 and -120 are actually different phases of an AC voltage wave. So by choosing which two points to connect to, a load can receive 120V or 240 V. Since there are two ways to get 120V, the breaker panel is usually set up to distribute the load between the two 120V phases.
As always, if you are in doubt about what to do, the best advice anyone should give you is to call a licensed electrician to advise what work is needed.
Before you do any work yourself,
on electrical circuits, equipment or appliances,
always use a test meter to ensure the circuit is, in fact, de-energized.
IF YOU ARE NOT ALREADY SURE YOU CAN DO THIS JOB
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.