Three words, almost interchangeable to a newcomer, that mean genuinely different things. They all involve extreme cold, but each answers a different question: how to make cold, how life reacts to cold, and how to use cold to preserve a person. Keeping them straight is the fastest way to understand what cryonics actually is, and what it is not.
Cryogenics: the physics of cold
Cryogenics is an engineering discipline with no biology in it at all. It studies how to reach and control very low temperatures, generally below -150°C, how to liquefy gases, and how materials behave down there. It is the toolbox: the cooling systems, the liquid nitrogen, and the insulated dewars that make everything else possible. If cryonics is the operation, cryogenics is the equipment in the room.
Cryobiology: how life handles cold
Cryobiology is the science of what cold does to living things. It studies how freezing damages cells and how cryoprotective agents can stop ice from forming and shredding them. This is mature, mainstream science: it is why sperm, eggs, embryos, and some tissues are routinely frozen and revived today. It is also where vitrification comes from, the technique of turning tissue into a glass-like state rather than ice.
Cryonics: the two, applied to people
Cryonics is what you get when you point cryogenics and cryobiology at a human being. It is the medical procedure of preserving a person's body or brain at cryogenic temperatures after legal death, using the physics of the first field and the biology of the second to hold the structure intact until medicine can repair it. Cryogenics supplies the cold; cryobiology supplies the method; cryonics supplies the reason.
The distinction matters because critics often borrow the credibility of one field to attack another, or assume cryonics claims to freeze people the way you freeze a steak. It does not. It is an applied field resting on two established ones, which is a more defensible position than its reputation tends to suggest.
Cryogenics makes the cold. Cryobiology explains how life survives it. Cryonics is the attempt to use both to keep a person whole long enough to be helped.