A Chiari (pronounced Kee-AR-ee ) Malformation is an abnormality of the lower part of the brain called the cerebellum. In a normal brain the cerebellum sits nicely above the bone at the base of the skull, if you have a Chiari Malformation you will have a herniation of the cerebellum tonsils greater than 5mm herniation.....in simple terms your brain is too big for your head and is falling down your spine! The Chiari Malformation may be corrected by a post fossa decompression surgery, this surgery will be carried out by a neurosurgeon usually at a specialist hospital. Drs Riveira and Pascual believe that the condition may affect 1 or 2 out of every 1000 births, (figure from 2007)
Chiari is not a simple condition, there are a large amount of symptoms that you may experience, some of which you may not link to this condition for a number of years and some of which may affect you physically and emotionally. The list of symptoms that may be associated with the condition can be found here. As you can see the conditions are extremely varied - you don't need to have all the symptoms to have this condition. You may think you have some of the symptoms listed but this does not mean you have Chiari, so don't panic!
There are also other conditions that have been found to be linked to Chiari which are discussed here.