Water is made up of molecules consiting of 2 hydrogen atoms and one oxygen atom, H2O. When you heat, and indeed boil, water you provide energy. When boiling the energy is sufficient to break the association between different H2O molecule, causing them to vapourise and form a gas.

Hence the bubbles are filled with gas. I am not sure if this gas is H2O in the gas phase, or if the energy provided by boiling the water is sufficient to break the bonds holging the hydrogen and oxygen atoms together. But I`d imagine it is H2O, as if not boiling water would produce two explosive gases as a product.

As for the second part, pure water has a defined boiling point, 99.7oC or their abouts, but if you modify the composition of water by disolving things in it you can reduce this. Therefore if you get close to boiling temperature, say 95oC, and add say salt, this might reduce the boiling point of the new solution, causing it to suddenly boil.

The bubbles in boiling water is gaseous water, otherwise known as steam. Pure water boils at 100 degrees Celcius when the atmospheric pressure is at one atmosphere. At increased pressures the boiling point is higher at lower pressures the boiling point is lower (like on the top of a mountain). Any addition of salt will increase the boiling point of the water.

Why do the bubbles form when adding noodles? Well that is a phenomena called nucleation. Even though the temperature is high enough for boiling, the bubbles need a nucleation site from which to form. Often the vessel the water is in provides the nucleation sites but many, especially glass vessels, have surfaces that are too smooth. Chemists use bump beads (glass beads) to provide the nucleation sites and sometimes broken pottery. That prevents their work suddenly exploding in their face. Care should be taken when removing hot liquids from a microwave oven for the same reason.

Why does the water boil when you move the pan? I suggest that the water was already at its boiling temperature and by moving the saucepan you are putting the extra energy into the system it needs to start nucleation.

One last thing. The foggy stuff you see above the pan is not steam. It is little drops of water that has condensed back from steam. Steam is a gas.

As every one knows the water we drink has a composition of organic content of which some of them are required and other which are not required(bacteria,virus,fungus)when we are boiling (if we go  deep into chemistry)the water some content is settled down these we call impurity but of solid type in the same way some bubbles(waste gases)arise but some of these have high density so they stay on the side walls of the container.And this also symbolize the complete boiling of the water.

these are some of the Question which arise when refering to the present Question.

What are scuba bubbles?  Why do bubbles float in water?  Air bubbles in sea float to top how big?  What happens to bubbles from scuba divers?  Why do divers release bubbles to the water surface?  Do bubbles emerging from a diver's suit become smaller?  Why do exhaled gases grow larger as they rise to the surface?  Why do bubbles grow larger as they reach the surface of water?  Why do bubbles increase in size when they float to the surface?  Small bubbles of air are released by a scuba diver deep in the water?  As bubbles scuba DIver would they get bigger smaller or stay the same?  Vapour bubbles in boiling water get larger as they approach the surface?  What happens to the size of air bubbles released by a diver as they rise?  Size of air bubbles in water increases as they move from bottom to surface?  What happened to the size of the air bubbles released by a diver as the rise?  Why does the bubbles exhaled by a scuba diver grow as they approach the surface?  Small bubbles of air are released by a scuba diver deep in the water As the bubbles rise they become larger smaller or stay about the same size?  Why do the small bubble of air exhaled by a submerged scuba diver get larger as they float to the surface of the water?  Why do small bubbles of air exhaled by a submerged scuba diver get larger as they float to the surface?  Why do vapour bubbles in a pot of boiling water get larger when they approach the surface?

Boiling water gives a physical change to water in the appearance of steam. This change in appearance contributes to the bubbles in the liquid form of water. Also, H20 is not the only ingredient in liquid water. Water can also contain Oxygen, Nitrogen and other gaseous elements. When you heat water, these elements will become active and be released into the air. 

Water molecules are held together by intermolecular forces.  At sea level these forces begin to break down as the temperature of the water reaches 100 C.  Except in very special circumstances water can not exist in liquid form at temperatures above this so at its boiling point a pan of water contains liquid and gaseous water.  The bubbles of water vapour are much lighter than the liquid and rise to the surface where a certain proportion condenses into small droplets of liquid water which we (incorrectly) call steam. 

It may be possible to superheat water.  I once microwaved a cup of water to make a hot drink.  I must have overdone it because when I added a spoon of sugar to the cup the crystals acted as nucleii for the water to vapourise and the eruption of bubbles dumped most of my drink onto the worktop.

Steam is the name for gaseous water which can be heated to incredible temperatures and pressures and is used to transfer power in industrial processes.  It can cause some quite horrific burns if it escapes from industrial boilers like those in power stations.