A stasis chamber (also called a stasis unit, stasis pod, stasis tube, or bio-pod) was a coffin-shaped device used to preserve life in critically-injured or terminally-ill patients until their conditions could be stabilized in a fully-equipped medical facility.
The stasis unit suspended all cellular activity and disease processes, keeping the patient from succumbing to their illness or injury for an indefinite period. The device was not meant for saving a dead patient. Time was required to get the ailing patient into the unit before they succumbed, presumably so that more time was available as they came out of stasis to effect proper treatment. (TOS: "Space Seed"; TNG: "Reunion") Stasis units became standard equipment on Starfleet vessels, including the Galaxy-class and Defiant-class starships.
The last of the Vaadwaur species placed themselves into underground stasis units, which they called bio-pods, in the 15th century to survive the destruction of their civilization. The pods malfunctioned, and so the Vaadwaur did not awaken until Seven of Nine activated the pods in 2376. (VOY: "Dragon's Teeth")
The subjects of a failed Cardassian experiment involving psychotropic drugs were left in stasis tubes aboard Empok Nor when the station was abandoned. The soldiers were accidentally reawakened when a salvage team from Deep Space 9 arrived in 2373. (DS9: "Empok Nor")
When Kathryn Janeway and Chakotay were infected with an insect-transmitted virus on a planet in the Delta Quadrant in 2372, The Doctor kept them in stasis chambers while he searched for a cure. After he was unsuccessful, the chambers were sent down to the planet so that the infection would not spread to the rest of the crew. (VOY: "Resolutions")
In late 2374, the entire crew of Voyager entered stasis units for protection, while Voyager crossed a Mutara class nebula that emitted lethal levels of subnucleonic radiation. Only the Doctor and Seven of Nine remained awake to operate the ship. (VOY: "One")