Sleep Apnoea comes in two forms. Obstructive and Central Sleep Apnoea. Apnoea simply means a cessation of breathing- you have stopped breathing like you normally do during the day. These breathing pauses generally come in two forms.
An apnoea is a total stop in your breathing.
An Hypopnoea is a partial stop which causes you to loose normal blood oxygen saturation and to briefly wake up.

Obstructive and Central Sleep Apnoea have different reasons for occurring but they have very similar effects on daytime functioning. Both of these conditions can only be reliably diagnosed through an overnight sleep study in a sleep clinic.

Essentially this is what they are:
Obstructive Sleep Apnoea (OSA) is a disorder of breathing while you are asleep. It is caused by a repeated blockage or narrowing of your airways. The severity of your OSA is usually expressed by the average number of these blockages or narrowings per hour (the Apnoea-Hypopnoea Index). These blockages/ narrowings cause your sleep to be fragmented from normal and can cause blood oxygen saturation drops during the night. During the daytime this will leave you tired, grumpy, less able to concentrate and prone to falling asleep (People with OSA have more accidents than other people). OSA probably causes high blood pressure and may be linked to diseases of the heart and possibly stroke.

People with moderate to severe OSA are usually successfully treated with:

Continuous Positive Airway Pressure (CPAP)- a small pump that keeps the back of the throat open so you can breath normally, CPAP has been proven to be the most effective treatment for OSA in most people.

Surgery -when something unusually large is present in the back of the airway and causing the blockage removing it can fix the problem.

Oral Device which looks a bit like a sports mouth guard and works by holding the chin slightly forward to reduce pressure on the airway while you sleep. Also shown to be effective in some people.

Weight Loss. Weight gain usually makes OSA worse, on the other hand loosing weight -if you are overweight- will usually reduce the severity of OSA. People with OSA should also try to loose weight in conjunction with one of the above treatments.

Central Sleep Apnoea (CSA). Causes the same problems in your life as OSA but is caused by something slightly different. OSA and CSA are not totally separate problems- people with CSA and OSA share some nighttime characteristics. Some people are neither CSA nor OSA but are somewhere in the middle and have what are referred to as Mixed Apnoeas.

Something in the brain-to-body connection is not working as it should. In OSA the throat muscles don't hold the airway open enough when breathing starts. In CSA the connection fails to tell the body to start breathing at all. CSA is rarer than OSA and thus is not quite so easy to study properly. Treatment for CSA is very individual, it depends on the subtype of CSA that you might have and needs to be investigated by your physician.

Other things that can make OSA/CSA worse are alcohol, sedatives (sleeping pills and the like) and always sleeping on your back.