Epilepsy is a neurologic brain disease which has seizure activity as its symptom. The seizure can be from mild to severe and may be proceeded by a sign (aura) from the dog, such as whining, pacing, or there may be no sign, or it may be missed by the owner. There are medications for treatment.
The very best articles on Epilepsy and its treatment options are at :
http://www.labbies.com/epilepsy.htm
Dr Dodds questions on the implication of frequent vaccination on seizure activity :
http://www.canine-epilepsy-guardian-angels.com/chang_vac.htm