Stars are formed from large clouds of gas. The gas, which might be the wisps of older stars which have exploded, or simply left from the Big Bang, is very cold and the cloud begins large but not at all dense.

Gravity, which causes all the atoms of the gas to pull each other together, makes the cloud contract. As the cloud gets smaller, the atoms of the gas get closer together and begin to bump into each other more, which heats them up. The cloud keeps contracting and getting hotter, until the pressure from the heat (which pushes the atoms apart) balances the force of gravity (which pulls the atoms together). When this happens, the cloud is a stable ball of gas like our sun, and is hot enough that it glows - it is a star.

Stars are formed from space dust and gases called the "interstellar medium". You may have seen pictures of it already. These are just dark clouds in space that are thick enough to see. It is so cold in space that the dust slowly comes together and after millions or billions of years the dust is so dense that the temperature causes it to undergo nuclear reaction and a star is formed.

A star is formed when a cloud of gas and dust in space has enough mass that the gravity from that mass pulls the material of that cloud to a central point rather than the particles of gas and dust escaping by their own velocity. As this material collects to a central point, atoms and molecules start bumping into each other. As more and more material collects, the material bumps together with higher and higher energy, making heat. With still further collection of material to the center of mass, the energy of hydrogen atoms bumping into one another becomes so great that they will occasionally fuse together to form a helium atom.

This process is called nuclear fusion and can only take place where there is a lot of energy to bring these atoms together. The process of fusion creates a tremendous amount of energy because a small amount of mass is converted to energy in the process and this extra energy causes more hydrogen atoms to fuse together starting a chain reaction that "ignites" the star. The energy that is now being produced is so great that it is radiated from the surface of the star as light we can see.

This new production of energy has atoms moving so fast that they no longer collapse under the force of gravity, leaving the star in an equilibrium between the force of gravity due to all of its mass and the force being produced by the atoms bouncing around with so much energy.

What do you think would happen if a star had still more mass such that gravity continued to draw more material into the core and even helium could undergo fusion to make bigger elements? At the other end of the scale, to make all of this happen, there has to be enough energy to make fusion start.

Some people describe brown dwarfs as stars that failed to get enough mass to start the fusion process and therefore ignite like our sun. What do you think a brown dwarf would look like? Do you think big gaseous planets like Jupiter could be considered brown dwarfs?

As probably covered in other responses stars are formed from clouds of gas that are big and dense enough to collapse due to the gravitational forces in the gas cloud. As the gas cloud collapses, the gas heats up and starts to radiate. In effect gravitational energy is turned into heat and then light. The gas cloud continues to collapse and the temperature increases until the pressure on the inside is high enough to counteract gravity.

If the mass of the gas cloud is low, less than about 10% of the mass of the Sun, the cloud keeps collapsing until the central core of the cloud is basically an extremely compact solid. This object never really becomes a star and gradually cools off. These are called brown dwarfs.

The planet Jupiter could probably be thought of as a very small brown dwarf.

For collapsing gas clouds with higher mass, the pressure and temperature in the center can get high enough so that nuclear fusion of hydrogen can start. The nuclear burning generates enough energy so that the star can continue radiating light away without collapsing anymore. Eventually though (after billions of years in the case of the Sun), the nuclear fuel runs out and the star starts collapsing again. Eventually our original gas cloud becomes a white dwarf (a hotter, more massive version of a brown dwarf) or does something spectacular like becoming a supernova.