Sleep apnea is a disorder where you stop breathing briefly while you sleep. An incident of the disorder, called an apnea (derived from the Greek meaning "to breath") is an episode where you stop breathing for a period of one or more breaths. While you sleep during the night, you'll experience many of these periods of stopped breathing. Medically, the standard definition of an apneic event is a minimum of 10 seconds between breaths with either a neurological arousal, a blood oxygen desaturation, or both. A sleep study, called a polysomnogram, is required for you to get a diagnosis of sleep apnea.
When you go to have a polysomnogram, doctors will define your disorder into one of three different forms, central, obstructive, and complex sleep apnea. In central sleep apnea, breathing is stopped by the insufficient effort in the respirotory system. If you have mixed sleep apnea, your breathing is interrupted by something blocking your air flow with your respirtory system still working. And finally, if you have mixed sleep apnea, your breathing is interrupted by a transition from central to obstructive features while having your apnea.
But, if you have sleep apnea, you probably aren't/weren't aware that you had it unitl someone else noticed it. Even if you wake up during an episode, you wouldn't notice that you were actually having sleep apnea. In a lot of cases, sleep apnea can occur for years or even decades before the symptons are discovered for what they are. Some symptoms that you could have due to sleep apnea could be daytime sleepiness and fatigue which is due to your sleep apnea episodes during the night.