Sleep apnea is when you either breathe shallowly or completely stop breathing during sleep. The most common form of sleep apnea is obstructive sleep apnea, or OSA. OSA is caused by a physical obstruction in the airway, typically loose or enlarged tissue or an enlarged uvula (uvulitis.) Airway muscles naturally relax during sleep; in certain circumstances they can relax too much. This produces snoring at best, and complete airway obstruction at worst.
This obstruction in the airway means that carbon dioxide builds up in the blood. After a certain point, the brain detects these levels and briefly awakens the person to allow carbon dioxide to be exhaled. This brief awakening is characterized by a choking or snorting sound that is easily noticed by sleeping partners (which is how most patients first seek treatment.) This shock disrupts REM sleep, the most important and restful part of the sleep cycle.
Though the structure and rigidity of the airway are often a product of the genetic lottery, certain lifestyle choices like smoking or diet exacerbate the condition. Obesity and allergies also contribute.