A star is a self-gravitating sphere of plasma in hydrostatic equilibrium that generates energy in its interior through the process of nuclear fusion. Energy radiates into space as electromagnetic radiation and neutrinos.
The sun is taken as the prototypical star (not because it is special in any way, but because is the closest and most studied star we have), and most characteristics of other stars are usually given in solar units. For example, the mass of the sun is
Msun = 1.99 × 1030 kg
The masses of all other stars are given in Msun.
Stellar evolution explains how stars are created and then die.
There are different classifications of stars ranging from type O which are very large and bright, to M which is often just large enough to start ignition of the hydrogen. the classifications are O,B,A,F,G,K,M. Our sun is a G2, which is very near the middle in terms of quantities observed. Most stars fall into the main sequence which is a description of stars based on their absolute magnitude and spectral type.
Stars spend about 90% of their life fusing hydrogen to produce helium in high pressure reactions near the core. Such stars are said to be on the main sequence. As these stars exhaust their supply of hydrogen, their outer layers expand and cool to form a red giant. Eventually the core is compressed enough to start helium fusion, and the star heats up and contracts. Larger stars will also fuse heavier elements, all the way to iron.
In smaller stars the outer layers are eventually shed, leaving the core, which is not massive enough for further fusion to take place and so is supported only by degeneracy pressure. This is called a white dwarf. The surrounding material stays visible for a while as a planetary nebula.
In larger stars fusion continues until collapse ends up causing the star to explode, sometimes leaving a neutron star or possibly a black hole. This is called a supernova. The outflow from supernovae and the stellar wind of large stars play an important part in shaping the interstellar medium.