You'll notice this happening on your thermometer. But at a certain temperature, the heating energy goes only to turning the water into a gas. When that point is reached, your thermometer will not get any higher and you will have found the boiling point of your water. Interestingly, no matter how much longer you keep the pot of water on the stove, it will get no hotter, because at this point all the energy from the heating unit is being dedicated to changing the water into gas and not into raising the heat. As you heat up the water, you are breaking down the liquid molecule so that it can turn into a gas.
Bubbles begin to be created in the base of the heating water first.
But the pressure of the outside air will squash those bubbles at first because they don't have enough pressure inside them to stand up to the outside air pressure. As more energy goes into making those bubbles though, they will begin to be able to stand up to the outside air presure. When they get to the point where they can stand up to the outside air pressure, you'll see massive bubbles coming off of your water, the temperature of your water will top off and your boiling point will be reached. I believe it's because the water gas bubbles have an easier time forming. The air pressure isn't quite as heavy on them, so they can withstand the pressure at a lower temperature, and, thus the boiling point is lower.