Cryonics is the only present option offering a chance at a much longer life in the future that is open to older and seriously ill people, the many individuals who do not have the time to wait for the arrival of rejuvenation therapies. Cryonics is a part of the broad field of low-temperature storage of biological materials, in this case the practice of placing humans and animals into that storage immediately following clinical death, thereby halting all biological processes and preserving tissues, especially the brain tissues that incorporate the fine structures presently thought to store the data of the mind. This is not mere freezing, which damages cells, but rather a process of vitrification that employs cryopreservant chemicals to prevent ice crystal formation and preserve the small-scale structure of tissues. This is known as cryosuspension within the industry.
Cryosuspension is undertaken with the expectation that advances in medical technology may eventually enable restoration of preserved individuals to life and health. For so long as the structure of tissue is preserved intact, it is possible that advanced technologies can one day work with it. Thus a small industry of cryonics providers offers cryosuspension services and a small number of patients take advantage of this in the hopes that future clinicians will have access to technologies for revival and repair, most likely based on applications of molecular nanotechnology.
This blogpost by Reason from Fight Aging! aims to explain the concept of cryonics.