Stars Are Self-Luminous Gaseous Spheres. "Self-luminous" means that stars generate their own light. How they do this is the subject of question number two.

Being self-luminous makes stars different from planets, moons, asteroids, and comets, all of which shine by reflecting sunlight. Stars are different in other ways as well. They possess far more mass, have different compositions, are hotter, and most are bigger than planetary objects.

The Sun: Let's take the Sun, which is our nearest star, as an example. It is a self-luminous, gaseous sphere. It has no solid surface. Its size is about 100 times that of the Earth and its mass is roughly 300,000 times that of the Earth. Its central temperature is 15 million degrees Kelvin (27 million degrees Fahrenheit), while its visible surface is a relatively cool 6,000 degrees Kelvin (10,000 degrees Fahrenheit).

The Sun is composed mainly of hydrogen and some helium. The heavier elements (carbon, oxygen, nitrogen, iron, gold, and the rest) make up only about 2% of the Sun's mass.

Some Properties of the Sun:

Radius = 435,000 miles 
Mass = 330,000 Earth masses 
Surface temperature = 10,000 degrees Fahrenheit
At center: 
    Temperature = 27 million degrees Fahrenheit
    Pressure = 300 billion Earth atmospheres
    Density = 160 times that of water
Current age  = 4.6 billion years

(The Sun's properties in metric units.)

The Sun's Luminosity: The Sun's brightness-- referred to as one solar luminosity -- is comparable to that of 4 trillion trillion one-hundred-watt light bulbs.

Other Stars: The Sun is often referred to as a garden variety star. This means that it's rather average. Some stars are bigger, more massive, and brighter. Others are smaller, less massive, and less bright. Some stars are single like the Sun. Others have one or several companion stars. Some stars are much older than the Sun. Others have been formed very recently.