Our Sun is a massive star that was formed about 4.57 billion years ago. In its core, constant nuclear reactions fuse hydrogen into helium and eventually produce solar radiation and neutrinos, or tiny particles which travel close to the speed of light. Unfortunately for earth and its inhabitants, the sun does not represent an indefinitely stable reaction, and over time its composition, activity and output will alter dramatically.
Many people imagine stars as dying through supernovas, gigantic blasts of energy and radiation; exploding stars. However, our sun does not have enough mass to end this way. It will instead likely become a Red Giant, or a large luminous star in the evening of its lifespan. In becoming a Red Giant it will expand greatly in radius, placing the earth well within range to be absorbed into and incinerated by the sun. There are many factors (such as the sun’s eventual loss of mass during its Red Giant phase, the outwardly expanding orbit of the planets, and the tidal forces of outer space) which make it difficult to predict exactly how the elder sun and earth will interact, but it is possible that even if the earth escapes direct incineration it will have all of its water burned off by the sun, effectively putting an end to terrestrial life.
After the outer layers of the sun become separated from its extremely hot core, the core will slowly cool over billions of years as it fades into a white dwarf. Human or other terrestrial life that hopes to survive beyond the natural lifespan of our sun will have to seek out new planetary systems to colonize!