What does euthanasia do?

Answer 1

Humans

It more-or-less painlessly ends the life of someone who is either terminally ill or in chronic pain or debility. In its modern form, it is sometimes termed "death with dignity" but may be legally classified as suicide, or homicide, when done outside professional medical practice.

Animals

Animals are euthanized (usually by drug sedation) when they are either critically or irreparably injured, or for stray animals, when there is no viable option to care for them. Euthanasia is common for severely injured pets because of the low probabilty of survival for any extended period, which would cause unnecessary suffering.

In early veterinary practice, there were several cases where attempted "euthanasia" by over-sedation failed, but resulted in the survival and recovery of the animal, once the stress of its injury was lessened. In humans, the analogous condition has become a deliberately-induced coma.