Primary seizures are unprovoked and not linked to an inciting event, while secondary seizures may be caused by factors like trauma, illness, or poisoning. Generalized seizures involve abnormal neuronal activity in both cerebral hemispheres, causing alterations in consciousness, and can include tonic-clonic, absence, or atonic seizures. Focal seizures originate in one cerebral hemisphere and may preserve consciousness initially but can spread and cause changes in consciousness.