Right now, the sun is a medium-sized star and is at roughly the mid-point of its lifespan. The sun is primarily composed of hydrogen and helium; the heat and light energy given off by the sun are the result of the nuclear reaction when "two hydrogen nuclei are combined to form one helium nucleus, releasing a tremendous amount of energy in the process." The amount of hydrogen that scientists estimate is still present in the sun would allow for this fusion process to continue for approximately another five billion years.
When all that hydrogen is exhausted, the sun will first become a red giant, expanding its shell and turning reddish in color. This process will be accompanied by the sun loosing much of its mass, and eventually any gaseous atmosphere will dissipate. At that point, the sun will become a white dwarf, with only the solids of the core remaining from the star.