“Anoxia” is a total absence of oxygen, while “hypoxia” is an insufficient amount of oxygen. For injury examples, drowning can result in anoxic injuries, because when submerged, a person has zero access to air and oxygen. A hypoxic brain injury could happen during a stroke, when oxygen to the brain is severely limited enough to cause harm.
Individual brain cells begin dying after 1 minute without oxygen, and permanent brain damage begins to occur after 4 minutes.