What is the largest difference between fainting and seizures?

Electrical: Fainting refers to loss of consciousness. This may be due to a variety of problems. A seizure may or may not result in a loss of consciousness, but specifically is a result of abnormal synchronized electrical activity in the brain. There are several types of seizures, only some of which result in synchronized muscle activity (often called a convulsion).

Brain activity: I agree with dr. Neish. I'll add that a seizure occurs due to abnormal brainwave activity. And fainting will typically have normal brainwaves. Fainting can be due to many causes: abnormal heart beat, low blood pressure, emotion, dehydration, low blood sugar, etc.