Cryonics (a.k.a. cryogenics) is the process of “freezing” a person so that they may be preserved until medicine and technology have advanced far enough so that they may be brought back to life. The basic principle is that the definition of “death” is dependent on the technologies of the time. For example, “A hundred years ago, cardiac arrest was irreversible. People were called dead when their heart stopped beating. Today death is believed to occur 4 to 6 minutes after the heart stops beating… Future technologies for molecular repair may…[make] today’s beliefs about when death occurs obselete.” 
Cryonics functions on 3 facts. First, life can be stopped and restarted if its basic structure is preserved. Even outside of cryonics, people have survived temperatures cool enough to stop the heart, brain, and all other organs for up to an hour. According to some lessons from biology, life is a particular structure of matter. “Life can be stopped and restarted if cell structure and chemistry are preserved sufficiently well.”  Second, vitrification (not freezing) can preserve biological structure very well. Basically, if one were to try to freeze a body, they would get exactly that: a frozen body. It would be swollen and filled with ice. Most of that 70% of water that makes up humans would be frozen ice. However, with vitrification, high concentrations of chemicals called cryoprotectants are added to cells so that tissues can be cooled to very low temperatures with little or no ice formation. Thirdly, methods for repairing structure at the molecular level can now be foreseen. As far as Alcor Life Extension Foundation  is concerned, nanotechnology is the most plausible solution for the success of cryonics. As long as nanotechnology keeps growing at the same rate at which it is becoming popular, it could lead to extensive cell repair and regeneration at the molecular level.
There are over a hundred patients at both the Cryonics Institute [4, 3] and Alcor , with the first case in 1967.  People are typically frozen at temperatures below -120ºC  or even -130ºC.  This is all done in the hope and anticipation that science and medicine will advance to a point where people in cryonics may be revived and able to continue their lives.
. “Cryonics: Alcor Life Extension Foundation.” Cryonics: Alcor Life Extension Foundation. http://www.alcor.org/. Web. 29 Sept. 2014.
. “Alcor: About Cryonics.” Alcor: About Cryonics. http://www.alcor.org/AboutCryonics/index.html. Web. 29 Sept. 2014.
. “Cryonics Institute.” Cryonics Institute. http://www.cryonics.org/. Web. 29 Sept. 2014.
. “About Cryonics | Cryonics Institute.” About Cryonics | Cryonics Institute. http://www.cryonics.org/about-us/. Web. 29 Sept. 2014.
. Woodruff, Bob. “Life on Ice: The World of Crazy Cryogenics.” ABC News. ABC News Network, 27 July 2011. Web. 29 Sept. 2014. <http://abcnews.go.com/Health/life-ice-world-crazy-cryogenics/story?id=14167348>.