Ever since the dawn of man the Sun has looked pretty much the same as it does today. This is because the Sun is a middle-aged main sequence star.
Virtually all stars spend the majority of their lives on the main sequence, until they eventually die; sometimes gently, sometimes violently.
It's All About Fusion
The definition of what makes a main sequence star is quite simple. When a star is forming, the material is clumping together, creating a denser and denser object.
The density in the core of the forming star reaches a point where the temperature will surpass about 8 - 10 million degrees celsius. At this point the kinetic energy of the hydrogen gas in the core is sufficient that the nuclei begin to fuse at an appreciable rate to balance the gravitational collapse of the star.
Eventually, perhaps over millions of years, the star becomes stable, reaching a hydrostatic equilibrium where the outward radiation pressure created by the fusion of hydrogen into helium in the core will balance the immense gravitational forces of the star trying to collapse in on itself.
It is at this point where the object is said to become a main sequence star.
The actual process by which stars accomplish this fusion is dependent on the mass of the star, stars below about 1.5 times the Sun's mass accomplish this by the proton-proton chain (protons combine to form deuterium and tritium, which then fuse to form helium and protons). However, stars greater than this mass limit use carbon, nitrogen and oxygen to drive the process (this is also known as the CNO cycle).
It's All About the Mass
So clearly mass plays an important role in simply driving the fusion, but mass is actually even more crucial in the life of stars.
The greater than mass of the star, the greater the gravitational pressure that tries to collapse the star. In order to fight this greater pressure, the rate of fusion must be sufficiently high.
Therefore the greater the mass of the star, the greater the pressure in the core, the higher the temperature and therefore the greater the rate of fusion.
As a result, the more mass that a star has the more quickly it will burn through its hydrogen reserves and the more quickly it will leave the main sequence.
Leaving the Main Sequence
High mass stars will leave the main sequence by becoming a red supergiant followed by a blue supergiant. The star will continue oscillating between these states as the helium fusion rate fluctuates. During these phases it is fusing helium into carbon and oxygen, and then these elements into neon and so on. The fusion becomes unrestricted to the core, where rings of heavier and heavier elemental fusion is initiated approaching the center of the star.
Eventually the star's core will be dominated by iron, the fusion of which is an endothermic process - that is, it takes more energy to fuse iron than the energy that is released. At this point the outward radiation pressure ceases and the star initiates a gravitational collapse, leading to a supernova event. Ultimately the core of these high mass stars will leave behind either a neutron star or black hole.
Conversely, stars with masses between 0.5 and about 8 solar masses will fuse hydrogen into helium until the fuel is consumed at which point the star becomes a red giant. The fusion of helium into carbon and oxygen ensues as the star becomes a pulsating yellow giant.
Once the majority of the helium is fused the star will become a red giant again, though even larger than before. The outer layers are the star will be driven into the interstellar medium, creating a planetary nebula. While the core of carbon and oxygen will be left behind in the form of a white dwarf.
Stars smaller than 0.5 solar masses will also form a white dwarf, but they won't be able to fuse helium due to the lack of pressure in the core from their small size. Therefore these stars are known as helium white dwarfs.